Let me approach this as an opportunity, if someone asked me to edit this (and certainly no one has), what I would suggest for changes. I'll split this into two main parts: "Minor Skirmishes" and "The Big Stickler" near the end.
Minor SkirmishesSome smaller things that caught my eye:
- Referring to "figures" as "stands". I think that initially 5E made the claim that it was trying to be compatible or support play in any traditional D&D system. If that's the case, it's a curious move to change terminology that remained constant up until now, as far as I know. The groups of 10 or 20 creatures throughout TSR's Chainmail/Swords & Spells/Battlesystem 1/Battlesystem 2 were always referred to as "figures". If multiple miniature figures were physically glued onto a platform together, then it was called a "stand". To this grognard it's pretty confusing to flip the two terms here in 5E.
- Mass scales seem OK. The figure ("stand") scale is given as 1 = 10 men. The distance scale is 1" (space) = 20 feet. That's exactly the same as in my Book of War game, and to my knowledge 1" = 20 feet had not been used before that point. So great. The time scale is 1 turn = 1 minute which is completely reasonable, and matches original Chainmail, etc.
- Grid seems like a bad idea. There was a time when I was designing Book of War that I gave serious consideration to using a fixed grid for the action, thinking it might simplify affairs. But I decided against it; while it might be arguably okay for independent fighters moving around, keeping large groups in any kind of formation on the grid becomes very burdensome. Movement and contact can basically only happen on the two axes, which gets more and more goofy-looking the larger your units are. But 5E Battlesystem does require the grid througout.
- Counting distance. The terminology here is that juxtaposed spaces are "adjacent", while a step further is "1 square away". That was hard for me to parse; the normal metric would be that the former is "1 space distant" and the latter is "2 spaces distant" (i.e., how much movement it takes to get from one to the other). Unless this is already standardized in 5E, I'd recommend changing that.
- Skirmishers vs. regiments. First of all, I've never really been fond of this distinction. It didn't exist in Chainmail/Swords & Spells (nor Book of War). It popped up in the original Battlesystem and I never really saw the need for it; it seemed like an unnecessary complication (particularly in the awful positioning rules that required a whole unit to be spaced out with figure 1" from each other; very inconvenient to move as a group). Secondly, the terminology has again been switched here: previously it was "skirmishers vs. regulars". The problem with "skirmishers vs. regiments" is that they aren't even the same part of speech, really: the former is a descriptor while the latter is a type of organization. Personally, I might cut the whole thing out for simplicity.
- Figures move and act independently. Something that I really didn't expect is that although figures (sigh, "stands") are assigned to mass units, in truth each one in the game acts, moves, acts, and defends independently. So you can't just push a big block of orcs across the table in formation; you're really obligated to decide on a path and goal for each individual figure, and just make sure that at the end each is still adjacent to someone in the same unit. As burdensome as this is, it's sort of required by the gridded space, because otherwise you probably couldn't get a unit to go anywhere except straight north/south or east/west. But it's aesthetically weird (and nontraditional) that you don't see or simulate any kind of consistent army formation. Attacks and defenses are also decided and resolved for each individual figure, which seems like it will take a long time. One advantage: it can support heterogenous units (some with axes, others javelins, or a mix using various at the same time), which is not something I could make happen with Book of War.
- Bookkeeping. This is one of those details that manages to quietly sneak in and you don't notice what a major deal it is until too late. ("A bit of bookkeeping on the side is also recommended," says a single sly paragraph on p. 4). In truth, because of the independent-figure action noted above, you definitely need to have every figure individually identifiable on the table (with marked letters or IDs, perhaps?), and then a complete list of every single figure to note its unit membership and its current hit points and conditions, at a minimum. This little "bit" is passingly similar to why I don't use Doug Niles' Battlesystem 2 (you need to match each attack die one-to-one to the figure who rolled it, which prevents rolling a big batch of dice at once). In contrast, I intentionally designed Book of War from the ground up to have no paperwork whatsoever -- the figures themselves on the table serve as all the record-keeping that exists in the game (with a few dice and markers). Every time we play BOW we set up our armies and then put all the paper away because we don't need it to play. In UA_BS you'd be shuffling lots of accounting paper, it seems. So you can consider your (and your fellow players') preference on that.
- Morale trigger. No morale checks occur until any unit is over half killed. While game-able, it's a pretty significant break to classical wargames, where some units will collapse unexpectedly from just a few casualties or the first brush with the enemy (which is arguably far more realistic). In Book of War we find that the greatest drama in the game is specifically from the morale checks that decide the fate of a whole unit; without that, you're basically in a repetitive roll/hit/damage cycle for a long time. So I would consider switching the rule to be more like AD&D or Moldvay B/X, where even the first casualty triggered a morale check.
- Attacks of opportunity. Wow, does 5E still have basically the same attacks-of-opportunity rule as in 3E? In retrospect I came to consider that one of the biggest blights on 3E; arguing about them was among the main barriers that turned my close friends away from D&D at the time. The main thing I notice here is that the "Reach" benefit again makes no distinction between reach from thrusting pole weapons (who can arrange together to catch opponents before they get close) versus giants creatures with clubbing attacks (who are presumably slow and swingy, and cannot just point continually in one direction at an enemy). That became as big a "proud nail" for me in 3E as the classic D&D missile range problems. I would recommend making the distinction.
The Big SticklerIn some circles this is the most contentious thing about Book of War, but failing to recognize it is so mind-warpingly, clearly wrong that I really can't understand how anyone can tolerate a game without it (link). There has to be a difference in the combat capacity of a mass unit, versus a solo/hero unit of the same type, because there are 10 times as many of the former -- obviously.
But the 5E Battlesystem draft rules have no such distinction. A mass unit uses mechanics exactly the same as in its normal D&D play. And without any other word on the subject, so does the solo unit. A single 4th-level fighter or ogre has 20 hit points and attacks twice a turn for 1d8 damage (or whatever it is nowadays). And a mass figure of 10 4th-level fighters or ogres apparently has the exact same 20 hit points, two attacks, and 1d8 damage.
One hero against 10 ogres is an even fight. And so is 10 heroes against 10 ogres. And so is 10 heroes against 1 ogre. If your warrior in normal D&D can fight off 20 orcs, then all of a sudden he can magically fight off 200 orcs just by virtue of playing a game on the Battlesystem table. Unless the exact same orcs are also declared as solos, at which point he can only fight of 20 of them again. Or if he's joined "magnificent seven" style by nine other heroes in a mass figure, at which point he can now only fight off 2 solo orcs (or whatever). Talk about Lovecraftian mathematics!
If we look at the tradition here, Chainmail Fantasy did not suffer from this problem, because the whole game was intended at one-to-one scale at all times (as opposed to normal historical Chainmail; link) -- although players who mis-read that game might be tricked into a similar scaling paradox. Gygax's later Swords & Spells did accurately and intelligently deal with this, although it was somewhat circumspect on the fact that in practice a solo hero would be easily overwhelmed by a single mass figure ("The hero will inflict .40 of the damage shown for a 4th level creature on the combat tables and sustain damage until sufficient hits are scored upon the figure to kill the hero", p. 1). Battlesystem intentionally inflated its solo heroes by about times 5 for gaming drama (or PC safety-bumpering, depending on how you want to interpret it), but was at least explicit about the fact that it was doing so ("From a mathematical perspective, the attributes of heroes in a BATTLESYSTEM scenario are inflated beyond those of the creatures in the units surrounding them", Battlesystem 2, p. 106). Looking further afield, Warhammer deals with the issue just like original Chainmail, by declaring that the action is all technically at one-to-one scale, so no mixing of mass-vs-solo ever occurs (6E, p. 279).
But the new 5E Battlesystem draft takes the cake for completely submerging the whole issue and thus creating an unprecedented full 10-fold power inflation due to the mismatched scaling (or really 100-fold difference if you flip solo-to-mass on both sides of a conflict). To avoid that craziness, I would highly recommend that the 5E Battlesystem rules recognize this distinction and fix it by either multiplying mass figure hit points and damage by 10, or dividing solo hit points and damage by 10 -- as I did in Book of War.
In conclusion: The kernel of the idea to 5E Battlesystem is fairly attractive and one I agree with: find a way to use the same basic stats and mechanic as in regular D&D for resolutions. But the majority of the design decisions run opposite to those I settled on for my game -- 5E BS being anchored to a gridded map, with heterogeneous figures that need to be moved, tracked, and resolved separately, and lots of paperwork for each figure's individual hit points and conditions. But overshadowing all of that is the fact that, as currently written, ignoring the distinction in scale between mass/solo units makes the game fundamentally unusable for the purpose of accurately simulating a D&D-system combat with larger numbers of combatants.