1: As many people have stated, it is relatively easy to obtain. However, the key point here is that you can't buy it from the Bazaar; you have to work to get it. And even if that said "work" only takes a few minutes of grinding, it's still work nevertheless.
2: Permanents are called permanents for a reason, no? And it is precisely the reason that they are harder to destroy than creatures that people incorporate Steals, Pulvy, and Explosion into their decks. They are VERY COMMON - no one can deny that. Every FG deck has a Pulvy. Speed fire often has Explosions. And any darkness deck benefits from steals. Only specialized decks, like pure speed decks, lack some sort of permanent denial.
3. Saying a Dusk Mantle "doubles the healing" simply does not make sense. It halves the offensive - so long as it does not get destroyed, Momentumed past, or is simply unlucky. Additionally, even if it DID double the healing to 60, common decks like Growth or Rainbow can still overpower it. The point is that it is useful to help you stay alive, but is not an "ultimate" card that will ALWAYS stay ahead of you.
4. The Heal spell and Holy Light are both instantaneous. They don't heal at the end of the turn. Reflection isn't a heal, either. Know your facts before you argue.
All in all, the SoGs are very useful permanents and may be powerful, but surely not OVERpowerful.
"Most variants on that deck will take out a FG here and there."
"Here and there" being the operative terms. Anyone want to try this deck? You will be utterly destroyed. And you can make this argument for ANY good cards in the game. An Oty deck will occasionally take down FGs like Paradox; a Speed Fire deck will, given enough luck, destroy Dark Matter.
"but your arguments are terrible."
I hate ad hominim responses.