They won't break a game, but they're better than your average cup o' dwarf, they hog the spotlight, and they require awkward work-around mechanics to be scribed into the rules themselves. They are exceptions to the rules, and that's part of their appeal but also part of their problem.
They preclude certain types of campaigns. In a campaign focusing on poisonous jungle snakes as adversaries (the Yuan-ti) or focued on spreading plagues, or focused on marine adventures, or focused on environmental dangers and the risk of starvation, they are *grossly* overpowered. They are immune to the campaign's main threat. Effectively, this precludes these types of campaigns for Eberron, at least on any significant scale.
Add in the fact that everyone wants to be one and that they *require* certain class abilities to function and that they are the best thing for drama queens since Drow, and they're just too much trouble for too little benefit.
Are android-people cool? Absolutely. But they're the kind of cool that works great in fiction but not so well in a game format (read: they make better NPC's). Either that, or they should be changed from Living Constructs to Constructed Humanoids, thus grounding their rules in a more inherently vulnerable type, which the game is already structured to handle. We don't need to give them 1/2 healing. We should take away their various immunities and make them more firmly like the humanoids. OR, we should restructure the game from the ground up and make monster types less mighty with immunities. Guess which one I'm more a fan of Eberron doing?
They're not going to shatter the game world at all. But any game that takes them into account has to make SPECIAL COMPENSATIONS for them all over the place. Can't do this, have to do that, invincible here, pathetically vulnerable there...all so that no one will play dwarves IMC again? Pheh.
They're a cool idea, but to quote Jurassic Park: "they were so busy thinking about whether or not they
could, they didn't stop to think about whether or not they
should!"