We started the Thaw campaign after about half the normal scenarios I think, which has gone fine so far (now halfway through that one). The encountered enemies haven't been able to fight back too much, but it's not like a few cultists are a real danger to even a starting band. In the end, you're mostly fighting each other anyway. The final scenario could involve more backstabbing rather than cooperating against the Lich Lord, which isn't necessarily a bad thing. Given the level of difficulty here and in normal scenarios (Genie, worm, a bunch of ghosts or 6 medium constructs are scary for a young warband too), I would actually simply say both an experienced band or a new one will work just fine. Would personally probably start anew, but that's mainly because after playing with several warbands there are still a good few spells I have yet to try out.