I don't think the ending was rushed, and I haven't read or heard of any behind the scenes issues.
I do know that a couple of scenes were cut towards the end of the movie due to pacing issues, and Stan Winston stepped in to do some uncredited VFX work in post production because Rob Bottin had finally collapsed from exhaustion.
Okay, the whole thing where Childs just runs off and disappears until the very end feels abrupt. In the original script instead of stupidly leaving him there ALONE while the rest go to check on Blair, they ALL go together and when they blow up Blair's ship they get attacked by him (it's dark so you can't see what he's turned into, just some monstrous form) and the line they're using gets cut and Childs gets tossed into the storm by the strong winds.
Then when they're in the Generator room they stupidly split up again instead of staying together and predictably this is why Gary is cornered and killed by Blair. Then Nauls just wanders off without telling MacReady (who is a few feet away from him) why he's doing so and this gets HIM killed. Then when the final Blair Monster emerges it...doesn't do anything. It just stands there and growls at MacReady instead of attacking him or knocking the dynamite stick Mac threw at him away. It just lets him throw it and leaves it there to set off the charges and gets blown up.
Considering the entire last act was different from the script when they get back from Blair's shack, they ran out of time/money and we got the rushed ending.