Obviously you would use Foul in Umbral Ice, since it doesn't gain any benefits from Astral Fire, would be silly to cast it in Astral Fire.
I think ppl are overreacting (as usual) from the potency nerfs, Yes F2 & F4 was nerfed by 20 potency, and Raging Strikes was removed, but when you look at the complete package it should be somewhat on par or even higher than on live right now. Also keep in mind that MOST dps jobs had potency reductions, its not only BLM, the sky isn't falling just yet.
