I have used Seamus' Avatar only once, but it was really good fun while it lasted
His fists ignore Armour so against Hoffman and his constructs, it really helped putting the Peacekeeper down! Causing Terrorifying to everything within 10" (including things normally immune to terror) can help eps as thats how he heals himself.
Cons - you do lose the Belle boost, and the very important Necrotic Ministrations. Nearly all of his buffs costs him WP so you have to be careful against WP heavy crews. I've said this on another thread, but Molly is very useful if your planning on Avatar Seamus as she can fill in the roles he can no longer do when hulking out.