I've found underworld to be especially rough to play for the first few games with no skills and can take some desperate moves to score. But once some players get developed they have a bit better time of it. The blitz'ers are great with easy killer skill access and stuff like 2 heads on the gobs make for a easier time.
Tactics wise i generally try to keep in mind a few things. firstly i found (cutlerys?) advice early in one of the bb threads useful - that you can to an extent control who and how many of your team gets bashed by who you leave next to the other team. underworld are weak str and av for the most part so keep them loose. goblins, especially with 2heads have a decent time dodging away out of zones. Try and get the troll to tie up as many players as possible giving you a number advantage on the rest of the pitch, him just standing there and not bashing can often be better than trying to hit and risking a bad roll/stupidity and tentacles works to that end.
as a playstyle i think trying to control space and movement is the best way to go against most teams (agi heavy its harder). try and restrict the movement of their players around the pitch by screening them out and try and be aware of the move speed of their players, if they commit their slow players too heavily to one side, push the other side and leave them in the dust, long run/hand offs are good for this as the play effectively moves to a different part of the pitch.
Defensively still try and keep your guys out of bash range unless totally necessary else you'll just get ground down. try and stall cages and let them only have the blitz to hit with, then try and make decisive strikes with your killer blitz'ers.
They're not awful at bashing against the high agi teams as you still get 6 skaven and a troll and the goblins are mobile and can move around for assists as long as your careful.
They deff feel like an advanced level team, you have to adapt to who your playing against, out move/dodge the slow bashy teams and hurt the agi teams. It helps i think to have experience with teams at both ends of the spectrum so you have a feel for both styles of play and move between them game to game.