Epidemius

In a nutshell: The Tally. That is all Epidemius is about. No upgrades, no CC tricks, no real shooting, no real surprises. He's a support unit, no more and no less.

Pros: He makes Nurgle units more effective in CC, especially those that don't start effective (namely Nurglings, Plaguebearers and to a lesser extent, Beasts of Nurgle). Nurglings are annoying with the Tally up, but the real benefit is to Beasts of Nurgle. They are survivable and more dangerous by far than Plaguebearers. The worst they can do (1 in 6 chance) is be as 'good' as they are. They are better the other 5 times.

Cons: Problem is, all the Tally does is make your marginally effective troops become more dangerous and survivable. The GUO and 3 Nurgle DP you are likely to run in such an army, don't really get more dangerous. The GUO gets a boost to FNP. Against a typical marine army, if you've killed 20 models...aren't you usually halfway home? Nurgle has low CC attacks on Plaguebearers, and you usually don't see them get into combat because they are slow and tend not to do anything once they are there. The biggest drawback is, the Tally does not improve your ability to kill tanks! So while your Nurgle Four Horsemen combo (the aforementioned GUO and 3 DP) might be nasty, they usually cannot bring the Tally up AND kill tanks. Especially as the most common build gives them Breath of Chaos to get Tally up quickly.

Tactics: Sit him in the back with a Plaguebearer unit as escort and ring up the tally with other units.

Final word: He can be effective in a properly built Tally army, but he isn't exactly going to do anything but the Tally. Don't forget that.