@Silverblaze: You're mostly right, but there are a couple of problems with what you just said.
First, making all powers available with various flavourings does not diminish uniqueness. No matter how you flavour Strength and Toughness, the dude with them is a tank.
Second, there's no danger to game balance from reflavoured powers. Because balance is mechanical, and flavour is flavour.
The danger comes from unintended interactions, which should be stopped with mechanical techniques. If ACaEBG is unbalanced when used with spellcasting (and I think it probably is) then not being usable with spellcasting should be part of ACaEBG's mechanics. Using narrative things to keep it away from casters is not a good solution at all, since players have the freedom to narrate as they wish. They shouldn't have to worry about having the mechanics crap out on them as a result.
Sometimes a mechanical limitation can look like a narrative one. As an example, I present to you the possibility that spellcasters be prohibited from taking ACaEBG. Looks narrative, but it's mechanical.
First: The more common powers are just that. Common. I'm referring to things like : holy powers, spell casting (which is borderline for rarity to be honest), shapeshifting, and powers listed only on items or NPC's in the Our World book.
Second:If you reflavour some powers they cease to be the same power.
Back to the narrative thing?
Fine.
"The player can narrate a they wish" - (I am tempted to say "No they can't" but I'll tone it back.)
Only within reason; within the confines the game set up in the rules. This is where the narrative and mechnics
blend. When a narrative starts to effect the mechanics (which it will in any game) and that narrative breaks the game. The player losesthe ability to narrate as they see fit. it ruins/breaks the game.
You see, here is where reflavouring becomes a problem. If a power is intended to be limited in scope by (narrative, theme, compels, or the rules simply saying - no) the book should say so. What we have here is in my opinion a series of mistakes on behalf of those who wrote the DFRPG.
To me it is clear holy powers should not be reflavoured to fit every character. They are unbalanced when combined with certain powers. Greater Glamours says only Fae may take this power. Why doesn't Sacred Guardian - ACAEBG - Super Potent Emotion - Myrk etc.? Lack of foresight? Assumption that players would not try to reskin them? Maybe in playtesting no one played Lenny the
mythic strength holy giant 1/2 temple dog with evocation and sponsored magic and 42 points of refinement who uses custom stunts and dual wields swords of the cross...because the character concept is rediculous.
The rules allow for as much freedom as you can take. Or as much freedom as you can hang yourself with. Depending upon how you look at it.
At a certain point you have to limit your players and yourself and say; this doesn't make sense. This will break the game. This should not happen. You need to look at context and say, this must have been an oversight in the writing process.
I've played in games with that kind of character. I've played in games where if everyone didn't do that they got sidelined or became wall paper. I learned how to do it. That is not fun for me. I don't play with people like that anymore.