Okay, I'm ready to revise my opinion on this, slightly:
With Battletech Advanced 3062 this is a legitimately good Battletech experience.
Tons of mechs, tons of vehicles, tons of weapons, full map, Clans, ComGuard, WoB, strategic conflicts -> megatons of content.
The skeleton of the mod, however, revolves around its reimagined combat model. So, in essence, to-hit chances have been radically revised (along with character perks), making positioning, sensor locks and - most of all - a variety of mechs absolutely key. Whereas vanilla wants you to use four Atlases as soon as humanly possible, BTA3062 will punish you for ignoring initiative and the to-hit differential between different weight categories. The latter basically makes it so that assault mechs are going to have a shitty, shitty time hitting light mechs, whereas the reverse is remarkably easy. The start of the career feels pretty brutal, with basic exchanges happening at 15-20% to-hit. This does get better reasonably quickly, but the first 5-10 missions can be pretty fraught. You really need to take advantage of evasion (the pips aren't peeled away by fire, so light/mobile mechs have a huge advantage in this regard) to ensure survival and victory. You've also got to be selective about what kind of contracts you take. Mountainous terrain, for example, is brutal if you're the attacking force.
Vehicles are also much deadlier. Just took out a Demolisher, for example, that could withstand multiple full mech salvos and toted a pair of AC20s.
Finally, the previous frustratingly excruciating load times and jankiness are somewhat less intrusive with this mod (I have no idea why). That, despite the fact you can now take eight mechs and up to three (or four, can't remember) vehicles into combat.
I'm finally having sustained fun with Battletech.