Thanks for the replies guys. (and the warning, sinker, 28 pages, damn)
I think the main concern the player and me by extend had was that it would seem to be very powerful effect in respect to some other aspects the book has. Something one of my players came up with was that it could be like a -1 power to create fallout, a bit like mana static that allows hexing.
For a more story based view. A power is something you can do. Its a ability you have, personally. Sometimes you are only conducting the power (sponsored magic) or it was given to you unwillingly, but using it in game is always the characters choice to employ it. The character decides to cast the spell or use his claws or whatever. Sometimes its the player accepting a compel but in game its still the characters choice rather then being forced (tough he might be acting according to nature rather then free will) This curse however is not a ability in that manner. There is no choice from the character to make it happen. The player could offer a fate point and ask if something wacky could happen but the character could do nothing more then pray for it to do so, he himself has no control whatsoever over it. I think that is an important distinction here, tough I can imagine somebody coming up with a list of powers that cross that line as well, I cant think of any.
As mentioned it also allows me to screw the players over royally and I could encourage the entire table to participate in coming up with the weirdest thing that could happen, so in the end if it makes the game more fun for everybody what's the problem?
I do think that with such a explosive effect I would want it to be part of his high concept tough. But that's a minor detail.