there are checks and balances to polymorph as an offensive and defensive spell.
if you use it defensively on an opponent's creature- it will bite you if he can pay to ramp it up (in other words is a bad idea). otherwise it is a solid soft cc spell.
if you use it offensively on your own creature to turn into the super bunny- it will only have a set duration, taking a card which would otherwise be overpowered into being situationally good.
*also, there is already +2|+2 included in the effect, no need for an additional +1|+1 from momentum.
*being able to finagle your way into having a perma-bunny is not desirable and means to do so were actively thought out and dismantled.
*its a short duration, decreasing survivability/attack strength further isn't warranted.