MarkB
Legend
In Next, that problem would be solved by using monster level instead of hit dice.
Depending upon whether the developers choose to do that. They could have done the same thing with CR back in 3.5, but they always seemed to feel that CR was too 'meta' a statistic to be incorporated into a game mechanic.
And it's still only a partial fix - the tricky part is that a special ability that's perfectly balanced when a monster gets to use it once or twice before having a sword rammed through its guts may be overpowered for a PC who can access it on demand by casting a spell.