As I said in the Reverse Time topique, Eternity is fine.
It's powerful but expensive to play and especially, to use!
Someone mentioned before, and I agree: sure,

gives most abilities to draw multiple cards in a turn
but Eternity runs for the very same

quanta, so, in a mono/duo

deck you're faced with the decision many times: draw an additional card, play a creature or use the Eternity... (In my opinion,

is a strong element but also an expensive one.)
In a Rainbow Deck, it's even worse, since there is no constant and reliable source of

quantum (even if your mark is

it gives only 1 quantum for sure). PLUS, the problem is the same: Hourglass(es) use(s) the same quanta. All in one, I personally ended up removing Eternity because in most cases I'm short of

quantum to use it reliably. [Though, as it comes to Rainbow Decks, I follow the good old "a fine card which I can play for almost sure is better than an excellent card which is unplayable many times since I don't have the quanta for it"]