I'd say it is 'hard-ish' and grim, the setting at the start explores the social, labour, and warfare effects based on the premise of a hyper-efficient space ship drive. So it takes travel times, gravity, economics, etc, serious, there's some heart-rending stuff in there.

I think people tend to call it hard sci-fi mostly because it is pretty rare for tv sci-fi to really explore the mechanics of space travel to this type of degree but there's also a lot of stuff it omits. I think the show is really good at taking a slightly fanciful premise and exploring it in quite a thorough way. Maybe I'm cooled on it now that it's been a while since I watched it but I remember when it came out it was just amazing to have a ship dock with a spinning space station and realise it thought it through more than I had.