Assassins used to take damage in the past from Stormvermin sweeps and should still take damage from ratling fire, but that doesn’t seem to be the case anymore when they’re pouncing.
I think this was changed with the Engineer patch or something? I’m not sure when it was changed but I haven’t seen an assassin getting killed by ff while pounced in a long time.
Maybe this is an unlisted Cataclysm feature though that’d be pretty stupid.