It's not true that there aren't any GBC hacks (Brown, Prism, XY Naturia) but most people feel like some of the later-gen mechanics (physical/special split, for instance) are essential features and Gen I and II make that a lot harder to implement than Gen III. (There's not enough move slots, for instance. Gen III has a limit of 511, but that's enough unless your goals are particularly lofty. Gen II has a limit of 255.)