I agree that it makes sense constitution would be the leading stat for Polymorph but I also agree it would just be too good. Benefits everywhere.
That being said, maybe a trade off could be that Polymorph skill could take direct hits at the health pool each time you use them.
If, instead of having it toggle based, they just make it cost some of your hp to use and was a timed skill and if certain benefits also come with deficits that affect non-skill based qualities(i.e. boost strength & slaughter move speed || boost dex & lower armor rating) then they can make it a little more balanced.
For skills that actually change the character's form (if they have those) maybe the character loses the ability to equip armor for that body part that could add a little bit of difficulty to the fights if it's something that might take away an important piece or something.
