Unless you play lots of Marines, I really don't think the Nurgle Prince is all he's cracked up to be - sure, Shrouded is probably the best bonus, but not being able to Sweeping Advance in combat is a huge deal.
Not sure I agree about this one - If you play the Daemon codex variant, you can get a Instant Death, Poison 4+ weapon, that is AP2 in the hands of the Daemonprince. With Initiative 8 and 5 attacks on the charge, that is absolutely devastating to any army that plays big nasties (Looking at you Eldar Wraithknights and Tau Riptides)
. In all honesty you won't need the sweeping advance most of the time he gets into combat.
The CSM variant has The Black Mace which is a whole other ballgame, seeing as it is one of the only weapons in the game that can hurt even gargantuans on a 2+, as they are not immune to fleshbane.