So my group used the original Unearthed Arcana Mass Combat rules from a year or two ago, and we hated them. There were lots of reasons why they didn't work, and this honestly changes none of them.
One thing that clearly jumps out to me is that spellcasters are still getting hosed (while melee may have it worse, let me explain)
So, you are the wizard, you jump into a unit, but decide to hold yourself separate from the guardsmen you are helping. Round X, you spend an entire minute casting a single Fireball spell... which is six seconds of you actually doing something. And the result is.... stop the battle, calculate how many creatures just got hit, have them all roll Dex saves as normal, okay these 15 creatures take 1/2 these 10 take full, that kills these 10, which then I recalculate the BR and reduce it (good thing you killed some or that would have done nothing to change the tide of battle) and... start back up the mass combat rules.
Why give us mass combat rules, and then tell the players that they don't interact with the mass combat rules?
I ran into this and something else while playing a storm sorcerer. Durations. I used Call Lightning (it was still in UA) and I would get... 1 strike every minute? That's 10 strikes over the course of the spell, when it lasts for 10 minutes though and I should be getting 100 strikes. And the wizard is only getting one spell off every minute, which is not realistic to their actual impact of casting 10 spells in that minute.
And how do cantrips play into this? My unit doesn't have cantrips, I do, but I can't roll against the enemies BR, I'm dealing damage to individuals hp... which has no bearing on their BR unless I actually get to kill them.
Moving on to melee characters... the 20th level fighter or Barbarian should be a terror on the battlefield like this. Over that minute the fighter makes 40 attacks, which (against minion style creatures) could actually decimate a unit singlehandedly, let alone if they have back up from soldiers... and how is this represented? It isn't.
In fact, I don't know exactly how to calculate CR for a PC, but I know it equals less than their level. An 11th level fighter with 3 attacks could kill 30 CR 1 creatures let's say. IF they are CR 9, they have a BR of 10 vs those creatures 15... which makes some sense because we know how a fight of 1 v 30 would really play out if we slowed it down, but in mass combat world that 1 minute of fighting leaves the fighter having killed... 4 creatures? If he rolls better than the enemy.
And this is even ignoring the weirdness pointed out up thread about LArger creatures being incapable of threatening smaller creatures because the BR, creature limit, and size limit on units, makes it so they have to divide their forces much more and can't be as terrifying as they normally would.
And, because I'm on a rant, let's talk range. If I am using a tactical map and make each square 100 ft like the UA suggests, then everything with less than a 100 ft range is fighting in melee, because they can't attack more than a single space away and the enemy moves an average of 300 ft a turn (30 ft times 10)....
Which is stupid. It means that an army can run up and smack a unit using bows, before a single enemy archer could make a single attack. How many times in history do you think an army charged an opposing unit, over flat terrain, who had ranged weaponry and suffered 0 causalities in the process?
And, any traps, hazards or defenses account for... what? What does charging and falling into a trench of spikes do to a unit's BR? Well, either you break out of mass combat to resolve the actions of 30 creatures being impaled on a pit trap, or you reduce it by 2... which could mean that your clever trap only slightly damaged the horde of orcs. Maybe you could do it by 5... but still that represents a very small number of combatants once you go beyond CR 1/4. Basic orcs are CR 1/2 right, so reducing it by 2 is you having killed 10, while 5 is 50. Fifty sounds like a good number of orcs killed in the trap, but if we bring it up to CR 3 veterans that same 5 represents 2.5 casualties out of 400.
And talking about switching between HP and BR, what happens when the BR is reduced. Let's say we had a mix of units, who gets removed? If they lose 5 BR did the enemy lose 50 orcs in that assault or 2 veterans? If the player decides to throw a Wall of Fire in the middle of the group, and you've got a mix of units... whose hit, who isn't? If the player doesn't specify or you don't have the actual locations of the various types of enemies planned out, then this is a waste because you could switch what the player was hoping to accomplish without breaking any rules.
TL;DR -> Inadequate rules that don't fix the major issues with the last time they tried this.
