Mostly speed. It comes out a lot faster, and 7 water is rather sketchy to get early without snovas. With snovas, you can sometimes get it really fast. Without them, you still can get it fast, but not AS fast. Even if you do get 3-4 towers on the draw, you still may not get 7 water for awhile since QTs are random. Plus fog shield does fine and can save you immediately. If you play permafrost the turn before you lose, you generally lose. If you play fog shield, you have a good chance of staying alive. And permafrost won't be nearly as effective in 1.25 due to the puffer fish nerf.
That said, it is certainly a viable option. It makes osiris an auto win basically, and makes Eternal Phoenix more viable, and would probably be better against paradox (though I see paradox as being easy anyway), and would basically shut down dream catcher.
Unupped forest spirit = pointless. You can get plenty of damage via mutated skellys, and if your bone yard is somehow destroyed you can rewind for indefinite amounts to win a war of attrition.