I'm almost completely sure bady part targeting is back for melee. No link, but it's confirmed (IIRC)
I think you may be confusing that with the special attacks melee weapons get in VATS.
The armor is supposed to support itself via servos, and it looks like there's collar or at least a pretty tight fit between the helmet and armor (it supposed to seal against radiation, right?). The joints are still going to be weak spots, but I'm sure you're generally going to be better off wearing power armor than not. Even without a neck brace, a football helmet will still protect your neck to some extent against a sharp impact to the top of your head by absorbing the blow over time as the padding compresses.
At the very least you'll blind them. Between their ears ringing from the noise inside the bell-shaped helmet and the fried optics they're not going to be able to tell where you are. Literally. And then when they release their helmet thwack 'em again.
As far as the armor supporting itself...well, if you jam the knee forward hard enough, the armor might interpret this as an operator command and might just fly out of control for a few moments.
There are going to be special vats attacks though - maybe some of them will specifically target certain limbs? It'd be cool if you could trip them or otherwise take out a leg, then watch them struggle to get up like a flipped over turtle.
If this is animated correctly I would die laughing and forget entirely what I was even doing.
And then do it intentionally, just to piss the brotherhood off.