That's what legendoftotalwar says, but a buddy on discord was just finishing his Nurgle campaign and told me that he thought they were pretty powerful. He said spreading the plagues kept his enemies pretty weak, but he also said he doesn't play on the harder difficulties. Legend said the plagues weren't all that effective on legendary difficulty so maybe the AI gets some protection against them on the higher difficulty settings. I found the Nurgle units to be pretty good in my Undivided campaign, though I didn't have a chance to use them all yet. Think I might do a Nurgle campaign next after I finish the ogre campaign I'm working on.