My players (level 12-13), through a series of... decisions, have ended up in a situation where they and perhaps a few hundred to a thousand allies will be facing off against a garrison of several thousand varied troops.
To resolve this, I was considering using the latest version of the Unearthed Arcana mass combat rules. However, mass combat is a significant departure from the usual D&D 5e gameplay, and UA rules have not been playtested and refined the way published rules are.
How well do these rules work in practice?
A good answer will draw from actual experience using these rules, and address the following criteria:
Time: One of the stated goals of the UA is to do mass battles quickly. Does it succeed? Are mass battles "quick," or do they seem to drag on forever?
Player Agency: Can the players make a significant difference in the course of the battle, in either direction? That is, if they make poor decisions, can those result in a loss, and if they make good ones, can they win the battle?
Complexity: This is a secondary issue, but how well did your players take to the new rules? Mass Combat rules are quite different from ordinary combat--was there a lot of stumbling over rules at the table?
For those considering a frame challenge, I'm aware that I can resolve this issue differently, without having to resort to a new set of rules, but I'm exploring this option specifically. Still, if you have any specific warnings, I'd like to hear them.