It sounds quite good for molesting smaller units, but i don't knnow how well it would do against a big block. You will need to cause 4 more wounds than the target to negate rank bonus of 3 + standard. Having siad that, with a high WS, decent toughness and -1 to hit, also with the impact hits, that could work.
I now have my own copy of the army book. I think that with flaming breath and a filth mace, run through something small before making a charge on a larger target and saving the flaming breath weapon for then, could cause some major damage. And then hed be pretty surviveable after that anyway? Hes a Nurgle Exalted Champ.