I think there's two types of remakes, and UFO Defence and it's remakes/whatever of XCOM, Xenonauts pretty much encapsulate them.
There's the Xenonauts route where it's basically "Remake EVERYTHING almost exactly, fix the glitches, update the graphics, but keep it almost exactly the core game." At this point, I can think of only two things different from the original (both pretty hefty in their own right, but still): You can't make money by building items and selling them, it costs more to build than you get from selling, so all your money comes from doing missions and keeping nations happy/funding, and
no psychic humans, it's a dead-end in the research tree last I saw
On the other hand, you have the XCOM style, where it's better to call it a reimagining than anything else. The... Idea of the original game, i.e. "Defending the Earth from Aliens, with Psychic Soldiers and a Hidden Base" is there, but so much is different that you can't really say it's the same game any-more, remade.
They can both work as long as the resulting product is good on it's own merit, and is a fun game, but they're very different design standpoints.
Edit: I dunno, Xenonauts seems fairly popular with the people who know about it, notwithstanding gripes about those two semi-major changes.
(The first one at least makes sense, you're playing a world-defense game, not an arms-dealer/manufactoring game*, so it makes sense to make your major resource depending on your... defense of the world. No comment on the second change)
It's not like it's reviled or has torn the fandom
*That said, I would totally play that game.