I don't play daemons yet, but looking at the Codex I'm just wondering if this tactic would work?:
Epidemius + something Nurgly with Breath/Wind of Chaos to cause loads of wounds and fire up Epidemius' powers + plenty of Nurglings to benefit from the 'ignores armour' result. Even at str 3, people have to watch out if they get no armour.
Also, I really like the concept and look of the SoulGrinders, but people I talk to seem to think that they're not very good - has anyone got any thoughts?
With Nurglings, you don't have to worry about the 3str. By the time you get to ignore armor saves you have noxious touch which wounds on a 2+ poisoned.
Epidemius is good for 40k (better than fantasy IMHO) but his army is really weak against heavy armor. Anything you put in you list that can handle armor well is one less nurgle model to contribute to/benefit from the tally. Also the play style is a bit different because you need to use Daemon Princes to cause wounds early on so your plaguebearers and nurglings are better in the end. If you risk losing your DPs and Epi gets killed, then you run the risk of being left with nothing but nurglings with no tally bonus.
Soulgrinders are situationally good. They are good in CC, decent at shooting, and will cost you less than a fully kitted out DP. If you get them locked in combat, they will survive a long time because the average model can't hurt them and powerfists only glance on a 5 pen on a 6.
Downsides are pretty glaring though. Possible one hit kill and they have a very large footprint so it's hard to screen them and nearly impossible to fully block LOS. I have a friend that plays them and when they come in, he has to put one of his potent HtH guys farther off into a corner because they are so large to reliably deep strike without suffering a mishap.