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. *snip*