So we went from jaw not working > jaw working > T-Rex outright missing? That's a real shame.
I'm of two minds on the Mosasaur scene. On the one hand, it was designed to fix major sightline issues that are only present on the USH version - and to that end, it succeeds admirably. It also better foreshadows the park's operational failures/the forthcoming disaster.
However, overall, I prefer USH's ultrasaur sequence... when it worked as intended. The Islands iteration cuts one of the ultrasaurs, and the sole beast's animation cycle starts way too late to be convincing. You aren't honestly losing much by putting in the Mosasaur sequence. (Islands would presumably get to keep its two Hadrosaurs if it made the transition to JW, to boot.)