Well, I know your problem, that's why I have two factions(Banebrood + Halodynes), so that I can borrow a faction to a friend. Or if there are four players, we play 2vs2, Reavers and Beastspawn against City Guard and Temple. Hell, that's fun.
Just to make sure, Mongrels deal good damage, it's just the have not that much MEL and their defense is low. If they charge for example my guthackers, you have to kill them, otherwise I need 3-4AP to wipe your unit away. And since Guthackers are immune to plagues, you can't really disable them. Since Banebrood posess a spell called Long-Legs, I don't see any great advantage using Mongrels. But if you know, what your opponent plays, Mongrels might be a good choice.
Your weakness as Banebrood are spectral creatures, since you have no valid magical option, with exception the cyclops! I ordered mine, so I can't tell, how good this creature is, but statewise, it will be hell for my opponents.
Last words, you picked a strong faction, which has interesting gameplay and cool minatures (the reason I picked them). GW is a ridiculous game, (I borrowed imperials) my general fled the first round, I played against a far more experienced player and had not the best dices, but still won, all thanks to the new codex unit, freshly released. So I will stick to Skirmish. Reavers are very funny to play, if you don't know, how to increase your Banebrood