The problem is probably two fold. MoN got buffed, but they also nerfed Shieldbros. They probably should have left Shieldbros alone and just buffed MoN and see how it panned out.
Well i mean, none of the accessory designs are final, neither is the Mark of Nurgle. I like that it has allot of HP (granted its a chore to heal them), though id really like that their HP gets a tad nerfed BUT they also get a small passive regen to HP, or at least to test how this would work in comparison to the latest iteration of the Mark. As for the downsides, i previously suggested two: 1) Impacting their mobility. Now even in TT they arent slower but we can make them kinda slower through design. If you ever played Planetside 2 you recall how MAXes need a second or two from a standing position to develop into a full sprint speed and do so each time they stop? Place that as a downside of the Mark of Nurgle. That way they can still keep up with the rest of the forces while still being less mobile in general. 2)Nerf direct DPS and replace with DoTs. Lorewise those who follow the path of the Plaguelord wear gear that is in horrible shape, since Nurgle doesent play well with technology, case in point: So we take from lore and turn it into a mechanic. Due to horrid maintenance of their gear their weapons do less direct damage (firing mechanism inside guns corroded etc) but do the remaining damage over time. For example a bolter does 20 direct damage, if carried by a Nurgleite it would do 10-15 direct damage and then apply the other 5-10 damage as a poison. This way they kill slower and die slower, which synergizes well with a tanky class that any class with a MoN should be.
There is definetly a difference between how many shots MoN needs to die depending on conditions like movenet and evading, but even with straight up charging the barrel number increases by about 20% (from 18 to chest to 22 to chest), so there is some bug, Michael is now aware of it i hope. Watch for Obbu thread about that in founders section.
So Chaos in matches look like this: 40% MoN, 25% Raptor, 25% havoc with PC, and the remaining 10 a mix of traitor tacs and AS. Only won fortress attack, since we can eventually whittle them down. Lost every other match. I personally died twice from actual bolts, and the every other death was melee, PC, or grenade. I did get some kills with bolts. You know...bolts? What is supposed to be the "standard" ammo in this game? I mostly play tactical, since someone has to actually cap on my team to have a chance to win, and it's not like I can play a SM and melee and cap, that's crazy talk.