I love the fact that UC & Kingmaker added Mass Combat to PF but, from the beginning, I've been plagued by nitpicks wildly varying in degree.
For instance, how do I resolve flanking maneuvers or mixed melee/ranged rounds with multiple armies on the field? How can I quantify player contributions to the battle in more than just scripted encounters? Why the hell is there no appreciable difference between a CR3 army of 100 L4 Fighters and a CR3 army of 200 L2 Fighters, except that the latter takes twice as much resource consumption? Why do I have to handwave things like "Heavy Infantry in the front and Spearmen on the flanks! Light Cavalry harass the lines and Heavy Cavalry flank the archers!"
TVLBaW is the answer to my prayers. Kenzie elegantly handles all of the intricacies of basic battle tactics and strategy while building on the existing system to minimize bookkeeping. It's not a perfect answer, and it definitely requires some effort, but it works if you work it.
Here's the way I'll describe it to my players the next time it comes up:
"Would you like to handle the next combat using the regular Mass Combat rules which you've seen before or would you like to use a much more fun system which requires a bit more time/effort? Basically, do you want to make mass combat faster or more fun?"
1) No, this does not have clear compatibility rules with UC Mass Combat. The pricing for units etc. are gp-based, which I will definitely NOT use. I plan to use UC's rules on army building/maintenance if I use TVLBaW for combat. I might houserule certain things with tactics etc, but even dropping it is a decent trade for the fun factor.
2) Some things are weird: A character avoids damage from a ranged attack volley with a DC15. Just a DC15 which never fails. Also, while DR is accounted for, regeneration is not. I'd probably treat that as the rules for healing, which don't allow dropped cohorts to respawn. Also, grab etc. has no effect. There's some good weird though: you can challenge an enemy commander to direct combat for three rounds (which takes place during a "paused" 1-minute mass combat round) and, if the commander refuses, their whole unit takes attack penalties.
3) It's very clearly designed for low-level play: My players have an army of 100 L5 Fire Domain Clerics which channel negative as a ray/smite and have Fireball as a 5th level domain spell. By these rules, once per battle they deal an automatic 1750 HP multiplied by every "cohort" of about 10 troops they hit with no saving throw (in this case max 9). An army of 100 level 5 Fighters with 40 HP each has 4000HP, and can be easily one-shotted even if you rule that it only affects 3 cohorts rather than 9. Might make more sense to rule that max damage is HP x size of cohorts affected. If a character is caught in the blast, though, they're guaranteed death even if they make all their saves unless they have Improved Evasion and lucky rolls. This is sticky stuff. Then again, it may say more about the current mass combat rules than anything else.
4) Surprisingly resistant to cheap metagaming: Remember that example of fighter armies above? The larger army may be fighting against roughly equal hit points, but they'll get an extra attack for outnumbering 2-to-1 and another if they set up a flank (which should be easier since they're twice the size). That said, they'll lose the latter advantage if they get bottlenecked and also get an extra attack against them if they get flanked themselves. Size actually matters now.
5) You're going to need statblocks: One of the things I appreciated about UCMC was that I could very quickly slap together an army on the fly without actually building out the base units. This system uses the actual stats and armament of the base units for combat, so you'll need to build a stat block for every unit type.