Regeneration Units in general are pretty crazy.
You can win most low-tier Vamp fights earlygame with just mannfred and the Vargulf on their own. Trolls at least get stuck if you sourrund them. Stuff like Vargulf and Terrorgeists will just casually plow through your line and go regen somewhere.
Unit hitboxes in general are incredibly f*cked up. Flying units will literally ignore your orders to disengage, if even one of them is still stuck somwhere. I've had a Unit of 12 Vargeists almost wipe after one of them charged almsot twice the distance than the others into a meele, and then couldnt get out. The other 11 just kept circling above the fight, getting shot at by ranged units, until the one guy finally died.
Units on the retreat on the other hand. Anyone ever tried to kill a retreating Lord in a Horse? You can literally cover a good 200 square metres around the guy in bodies, and he will casually walk out of it. Most units just run alongside retreating stuff if you tell them to kill it, with single units being the absolute worst offenders. I've had fights with 2 Warbosses and a Goblin Big Boss, where I told all 3 of them to kill one retreating Warpriest. The guy was on FOOT, and all 3 of them had mounts. They just kept bumping him to the ground without doing damage, and then walked him to the edge of the map with a good 1k of hp to spare.