Then it's the flip side problem. A spell that grants hundreds of HP at zero cost, typically greatly increased combat output and quite probably considerable movement bonuses should probably be an 7th or 8th level spell. Or to put it another way, a 5th level spell that lets my 9th level wizard make the 9th level fighter look like a complete chump is a problem.
Personally? I'd rather they made it simpler. You gain +2 to hit, +5 to damage and 10 (or maybe a bit more) temp HP/round. Choose a movement type that makes sense and off you go. Flavor what you turn into any way you like. On the flip side, as a spell against baddies, you simply remove all attacks and abilities for the duration, save every round. Reduce speed to 5 feet.
In both cases, the target loses access to class/race abilities for the duration.
There, now we have a nicely balanced spell that isn't ridiculously over powered. Tweak the above numbers to fix it better - that's my back of the envelope numbers. Clean up the text and poof, done.
But, of course, that will never happen because heaven forbid that we actually have clear rules.