This is actually one of the few good things about the new Mordheim game - the campaign management.
Unit evolution is definitely incremental, but can be significant in the long term. So, say a unit can level to about level 10, and two or three times, you'd get extra action points. These jumps make quite a difference tactically. Furthermore, there are tinier increments, like better hit chance, better armour, better chance to crit, etc - which is already in Battle Brothers.
The way I think things balance out is that in Mordheim, units can become less valuable over time too. So, for instance, if someone is KOd in a mission, they might not die, but might get debuffs, like losing an arm (arguably good sometimes, because it gives you +20% dodge
), losing an eye (reduction of hit chance), etc etc.
So, your warband is generally built out of three tiers: the greenhorn cannon fodder, a mid-tier of troops that have levelled a bit and have a few useful skills, and a core of experienced units that you are very attached to that you protect carefully and that die rarely. Over time, they will get debuffs from injuries, and will become less valuable. At some point, they are no longer particularly good, and this is when one of the better mid-tier units comes takes their place, maybe by switching some equipment over, etc.
With careful balancing, this system can be very compelling (also frustrating). The pro is that that it doesn't give you an end-game where everything is trivial to beat... you are a perpetually changing team of mercenaries, and as you get better, hopefully you'll move more and more units to the top tiers. The con is that it doesn't give you an end-game
You never get a steady-state of being awesome. You keep losing and getting new people... and may stop feeling like there is progression.