I agree with the OP. I have used polymorph and have not found it that insanely powerful. You can make the fighter obsolete, but so can lots of things casters do, like summoning, or CoDzilla. UNLIKE those things, however, you can actually share polymorph *with* the fighter, as it is not a self-only buff, and instead of replacing him, use it to make him more awesome. That's actually my favorite thing about the Polymorph spell.
I do think out of core, there are a lot of really broken monsters, and there are feats and ways to become different creature types to expand your options beyond what was originally intended, and that can break polymorph. But it does take effort, cheese, and blatantly unbalanced monsters. In core, you can best use it for the hydra form, yeah. But you wouldn't get the single-minded coordination to move and attack with all the heads, that's clearly a "quality," unlike pounce, which is an attack. I guess Cloaker can also be abusive. That's...about it.
I think Enervation spam is more powerful, really. Toss a lot of those (empowered, split, quickened, and/or maximized if possible) and you can ruin a caster pretty fast. As casters are the most potent threat in the game, that makes it a pretty powerful spell.
Shapechange is definitely broken, though, no question. And Alter Self is way too good and long-lasting for its spell level. If higher level and nerfed a bit, it'd be fine. Both are also self-only, which makes things worse.
But Polymorph itself I've never really had an issue with, I think the internet condemnation of it is way overblown.