Like my other feedback thread, this thread is being written with a light tone. I can't believe I have to have a disclaimer, but time and again I've had my feedback taken... poorly... in games because people are incapable of parsing subtext.
I'll start with the Reborn Marauders, as they're the most common nurglite unit I come across with the Nurgle mark heal, and the most problematic through the whole game. First, remove the absurd knockdown immunity. Champs in Van Helsing weren't immune to CC, they just suffered a shorter effect, and that's fine. But between the knockdown/stun blanket immunity and the fact that they will arbitrarily warp through terrain with their pull technique, I find that I'm burning a LOT of cooldowns when more than one is in a room. Usually, I will trigger something like my shield rush and I'll get locked in place, wasting a mobility tech and leaving those pesky ranged mobs (ref: Shield Discussion feedback thread) alive and able to continue carving chunks of my health off. In cultist only investigation missions, if there is one in the room I average losing about 10k EHP (between shields, health, and regenerated health). If there are two more more, I will die.
Why? Because they tend to surround you, cannot be jumped over, cannot be bowled through, and each one does (with 35% damage reduction) between 500 and 1500 damage per hit. This is simply not feasible to tank through, even with fairly "decent" gear. This feeds into the next point--their damage needs to be nerfed to about 1/3rd the current value. Alone in a duel it's a challenge, in a room with three to six of them as a melee character with no dodge, just "mitigation" (again, see other thread), it's a brutal slog. Three of them can clear my healthbar while I'm in an animation loop, and on top of it in three hits drop my suppression into the red, stunning me and killing me. Not fun, and more "boss" ability than Elite/Champion mob ability.
Now we arrive at the Mark itself. The regen on a Marauder outpaces most of my melee abilities. If I have three Marauders on me, I might be able to kill one, but the other two will regain full health between cleave swings. Again... with bonus damage from skill points, mobs will heal to full in the time it takes a chainsword to clear the 1.3 second attack CD. That's... insane. I'm not expecting to cleave through them like trash mobs, but I am expecting that my time isn't wasted, and with how many of them get thrown at me to a room, that's what it currently feels like.
Now, this is extra !!Fun!! (dorf fort, yes the flame tag is intentional) when applied to, say... those beastly Daemon Engines, who have really bizarre leash lengths for aggro (that seem to change from pull to pull, even with the same engine on the same map). I've had a dozen Nurgle dreadnoughts heal over the damage I could put out with an alpha strike (highest damage, firing everything at once), making them an intense slog to fight... and then I get them down to half health or lower, and it suddenly decides it has something more pressing to do, wanders off, and is fully healed before it stops plodding. !!FUN!!.
The key to handling health regen on enemies, like drinking tequila, is moderation. If an appropriately skill/level/geared character has difficulty taking on a common enemy (regardless of the Champion/Elite tag, if I can run into 5 or more in a single room they're trash mobs with a fancy title), then it could stand to have a wee bit o' rebalancing.
My suggestion? Since it seems far less of a slog in coop, perhaps do what you did with Van Helsing, or what many devs have done with ARPGs... pin down a solid solo value and have a stacking +% bonus to mobs per player added. It's worked well for ARPGs (even those intended for party play, with solo being largely an always-online afterthought for those of us with bad connections) for two decades, I can't see it hurting here.
Edit: Strictly speaking, not entirely relevant to this thread, but I would also recommend switching the buttons for Scything Strike and Eviscerate on the 1h chainswords as well... the cooldown for Scything Strike is short enough that it's the default attack when dealing with most rooms, and it just seems like it's the (unintended) primary attack for the weapon with Eviscerate being the (non-stunning, for some reason) backup.