Tell that to the nymphomania deck ;]
Nymphs are clearly less used than other cards since they are rare and even having 2 or 3 copies of the one you need is unprobable, so it's hard to make decks around them.
They are also not designed to be more powerful than other cards, but rather a powerful ability-on-a-stick balanced with high cost.
Still, some feats are hard to achieve without a nymph. (eg. gravity nymph - quanta lock, auburn nymph - earth squid style or armagio style, fire nymph - CC-on-a-stick)