They may have reduced it (patch notes say so), but on a mission with 150 pr above mine (borderline "red),
The Nurgle Nests did have some very high regen,
it was doable for me, but I dare say my dps is quite good, and any char with just enough dps to run this mission, would have been unable to kill the nests.

As they already spawn constant streams of new mobs, I recommend cutting their regen down to non-serious levels, so that the regen will only be a problem if you need to retreat for whatever reason.

Instead, boost their HP a good bit so they still don't go down easy.

That should do it.

well, no need to think of it any longer, looks like it's fixed. ^^

Accidentally downvoted but +1 on that, getting those missions in Tarots is essentially a waste of fate because it's impossible to kill them.

Tried to do a "clear the nests" mission 140 rating above mine (sniper assassin). Was going fine, slow but steady, then I get to the first nest. Clear the mobs and start shooting the nest, can only damage it slowly, when I have to reload, it regens to full. Had to surrender. So much for challenge yourself. They have mechanics that make mission objectives impossible if your rating is too low.

I ran into it being way too fast in multiplayer, but in that single player mission with two nests and the demon I was able to take them down pretty easy (as a sniper).  It was kiting all the crap they were dropping that made it hard. 

Agreed, Nurgle's miasma regen needs looking at. The demon boss around campaign mission 8 or so is impossible to kill.