Prove it.
It isn't. It's not overpowered at all.
It makes any card overpowered except aether cards
First of all, only creature cards. Second of all, only cheap cards, unless you have a whole ton of quanta. Getting that quanta is probably not going to make as fast a deck as a good Shrieker, mono-Life, Golem, or mono-Fire.
it makes the life rush even faster and more lethal
Actually, this is false. A normal life rush deck (Frogs, Adrenaline) is faster than a Frogtal or a Cockatricetal. Try it yourself before making a judgement call.
ever fought eternal phoenix?
Yep. And he's one of the harder gods, too.
Ever fought Rainbow? He's even harder. But does this make Lycanthrope, Quantum Tower, Eagle's Eye, or any of the cards he uses overpowered?
included in any rainbow deck it can have practically infinite uses without even being considered important enough card
First of all, "any" rainbow deck is simply false. You need lots of
in order to fuel Fractal, and a tight speed rainbow simply won't have the quanta to play it. Secondly, "infinite" is also a false statement. It will have a finite amount of uses.
Basically, it's a very powerful card, but it requires high specialization and lots of quanta to play. Even a Rainbow deck needs to be specialized in order to use this. Your arguments don't convince me.