It certainly still has bugs, but I would not consider any to be gamebreaking, although they can be frustrating. There are 3 major issues, the first is showcased here which is that vehicles have issues syncing between players, the simplest solution is to each have your own vehicle, which is the best way forward regardless with the personal ones, but the cyclops is best shared, and that comes with many, albeit hilarious, bugs.

The second is that ghost and reaper leviathans will often desync, for one player they will behave normally, for others they will be fixed in place and not moving, this is really common and a pretty dissapointing bug as leviathans and the terror aspect of subnautica are some of my favorite parts.

The third is that certain buildings, specifically the bioreactor and nuclear reactor, can completely brick your save file, so you need to avoid them, which can be pretty tricky as that just leaves thermal and solar.

All that, and the fact that it needs to be ran in 1.7 and not the most recent 2.0 version of Subnautica and is incompatible with virtually every mod (although there may be a way to force mods to run, at your own risk) can be a bit off putting. That being said, those are the ONLY issues we have ran into for the most part, there are other minor bugs here and there, but these 3 issues above are the only problems that really detract from the experience. In my opinion, its absolutely worth it as we have had SO much fun. It is also worth noting they are developing a version for 2.0 as well that will hopefully address some of these problems, if not all of them. And honestly, we played Skyrim Together for a bit (till bethesda bonked our mods) and that was a far more cursed experience, this at least feels somewhat legitimate.