It's at the hard sci fi end of the spectrum, compared to Star Trek and Star Wars, but not as hard edged as TE, but get close. They have FTL and artificial gravity, but the space combat portrays pretty decent zero-g manoeuvring and combat tactics. Some of the space battles are among the best ever committed to screen, and the most realistic before TE anyway although some Babylon 5 fans might argue. The CGI was a good generation or two better than B5 though and still looks great.
There is some science fudging for sure, and I have to warn you later on it gets more and more metaphysical and ambiguous. The last season turned some fans off, but it really didn't bother me. I'd have rather is stayed more grounded, but even right to the end the characters and the storytelling carried me through. Some of it's tough watching, a few of the characters really go through the grinder.
I'd highly advise checking some clips on Youtube, and maybe check out the Spacedock channel's analysis of some of the battles.
If you like anything in the spacefaring sci-fi genre and haven't yet watched the 2000's iteration of Battlestar Galactica, you're in for one hell of a treat.
Be forewarned. It is very difficult to avoid binge-watching the show.
I did like Expanse, but I tried to watch a few of the older Star Trek and/or Star Wars movies (don't remember the difference), and I didn't like them at all.
