I don’t know if you’re exaggerating or I’m just lucky, because I never have issues as bad as you described. If I’m client and connected to a laggy host then there will be some weird laggy rubber banding and clipping, but if I host it’s generally fine with occasional manageable weirdness. Hyper density happens sometimes for sure especially where climbing up or down is involved but there’s a lot to be said for proactively preventing hyperdensity. Seeing the places it will form and thinning enemies out or just getting yourself somewhere else and all that.
I’d be curious to see how full unit collision would change the game. As frustrating as it is, hyperdensity serves the function of you getting overwhelmed by the massive horde… with unit collision, way less enemies could get at you at once, and as long as you weren’t mobbed completely out in the open the game would be ridiculously easy.
Similarly with the examples you mentioned of assassins and packmasters being able to just pounce or run through other enemies… if they couldn’t, they wouldn’t be threatening in the slightest. If a packmaster had to wait until you killed enough of the horde for it to have a turn, it would just be standing in line like an idiot and easy target. If it did manage to hook someone, it wouldn’t be able to take them away because it would be blocked in, so still easy target. Same goes for assassins.
Maybe some sort of soft collision for horde and elites would help. I haven’t played left4dead in awhile but it had similar mechanics with special enemies being able to pass through other mobs, and the hordes could form hyperdensity but it was less common - really only happened if you got yourself into a bad position. Of course in that game you had unlimited pushes so that was pretty effective at fending off the hyperdensity, plus the focus was on ranged combat so there wasn’t as much opportunity for it to form in the first place. Still, I think it says something that a big studio with endless resources still didn’t go the full unit collision route…