It varies depending upon what you want in your campaign.
Since most of my campaigns are VERY epic in scale and tend to be inspired from videogames, I rarely have a problem with some 'ultimate' combo that trumps others.
Though I do like to make all choices equally as valid...
For most campaigns, that's not really acceptable.
FR is a smidgen over the line of what the Core Rules generally allow. Not game-shatteringly....nothing in FR will make the game unplayable...but it's enough to make things 'easy choices' or 'no-brainers'.
AU is more significantly over the line, but it assumes you won't be using the Core Rules anyway. AU is close to the realm of game-shatteringly, in a few cases. But if played with at it's own level, it's perfectly fine. It would make your halfling rogue a bit irked to see a quickling faen akashic with two feats at first level, though....the halfling rogue is just WEAKER by comparison.
But in FR, that halfling rogue may just have an extra +2/+2, or other minor something, that the GH halfling rogue doesn't.
Which is why I allow in things on only a case-by-case basis. Still, I'm generous. I always think that no matter how good the PC's roll and how great their combos are, I can challenge them...it's just about thinking bigger and tougher than them.
