Obryn
Hero
Any specific "flaw" system of the "take a flaw, get a free feat" is, indeed, immediately abusable. You can either build your game with this in mind (like Savage Worlds) or you should probably leave it out.There has, to my mind, never yet been a flaw system for an rpg that I've encountered that isn't immediately easy to abuse horrendously.
On the other hand, "Trouble" Aspects, like in Fate Core, are pretty much the opposite of abusable. Ideally, there's an upside (invoke) and downside (compel) to it, but if you're only getting your metagame currency when it actually causes your character problems in play, it's a much bigger presence in the game and basically impossible to munchkin out.
It also leads to better roleplay. The example I saw is with a flaw like "Alcoholic." If you're playing GURPS, that flaw will encourage you to play like the opposite of an alcoholic - staying out of bars, never touching liquor, etc. You might as well call the flaw "teetotaler" at that point. If you're playing Fate, though, you're more likely to spend a few hours doing stupid things at the bar, getting yourself into trouble, being late for work, and generally acting like an alcoholic to get your points.