As has been stated, the main issue with hydras as iconic creatures has been that they're all generally big beastly ground beaters that almost all follow the pattern of "Enters with and/or has a way to gain +1/+1 counters. The method of gaining +1/+1 counters may also do something else and something else counter related may happen." which, honestly, gets a bit dull and has them all start to merge together.
On the other hand, angels and demons and dragons do all sorts of varied and neat things that range from more practical to super flashy.
Add into this the fact that, other than Protean Hydra, I feel the hydra most dead on flavour-wise remains the original one, the mono-red Rock Hydra, and it's all a bit weird.