But that's not even the most severe design regression. The T-X, like Schwarzenegger's moldy old T-800, has a physical endoskeleton. If Skynet can make a time traveling assassination machine out of liquid metal, why would it ever build one out of anything else? The T-1000 could not be harmed by anything -- it shrugs off bullets, grenades and explosions without a scratch. Arnold takes the same abuse, but comes out looking like he got raped by a Tyrannosaurus, because he has a permanent solid-state structure. So why would a sentient supercomputer overlord read the performance reviews of those two models and decide "Whoops, scrap that 'mimetic polyalloy' bullshit, clunky metal horror-bones are definitely the way to go"?
Yeah, that's much, much better.
The T-1000 essentially has no limitations as an infiltration unit -- as we've already mentioned, it can mime pretty much anything, but beyond that, it can get in and out of any place it wants simply by engaging "puddle mode" and sliding through whatever narrow entrance point is available. It wouldn't matter if the only way into a building was a tiny hole in a 10-foot concrete wall, it could just turn into liquid and drip the fuck in. But the T-X, which is supposed to be the T-1000's upgrade, has all of the physical limitations of a standard non-robot person. If there's no door, it has to either bust a window or gas up its breasts and try to jiggle its way through security. The T-1000 could come in through the damn water fountain.
Finally, when the T-X has its climactic battle against Arnold, this happens:
"Shit, I'm stuck!"
"Oh, wait, I forgot -- you're totally not invincible."
That's Arnold grabbing the T-X by its spine. He holds it in place long enough to detonate his own power core and destroy them both. It is literally the only reason the evil machine gets defeated -- Arnold is pinned by a door, so if the T-X didn't have a physical structure for him to wrap his meaty cyborg fingers around, he would have been helpless to stop it from killing his BFF John Connor. For comparison, here's what happened when he tried that same nonsense on the T-1000: