I'm going to post a scenario that I'm sure will happen and that I personally think is unfair but inevitable. This isn't a criticism of the whole Hive Memory system, I understand why it is being introduced - one of the biggest complaints people have about playing xeno is 'waiting to mature'.
The scenario:
Elder Boiler (666) gets careless and dies in an OB. Mature Spitter (325) evolves into Young Boiler (325) and picks up bombarding marines. 5 minutes latter Bloody Larva (666) bursts out of some body back in the hive, sees that there are no T3 slots open and demands the queen devolves 325 so they can come back as a Mature Boiler "which is better than having to wait for the young one to mature".
I think one solution to this is excluding T3s from the argument. You can only come back as a T2 at most.
Although I will say that I'm still a big fan of just flattening the power curve, make young better and ancient weaker so that waiting to mature has less meaning. I'm also assuming that the way Hive Memory works here means you can't evolve to your old self (if you were a T3) if there are no T3 slots open, if that's wrong ignore my scenario.