I've never played this but have been aware of it for a while. Judging from the Readme it sounds fairly complete? Like I could play out a full scenario in this without missing features or crashing?
I contributed a little bit back in 2014/2015 and it was a lot of fun. Because it is almost all hand-written x86 ASM (which is insane), it was actually not too hard to reason about, certainly far easier than what Clang/GCC will spit out.
The game works really well. I don't remember noticing that anything was still missing in singleplayer. Multiplayer... there was something, but I don't remember what. Desyncs for sure, but I think those were always solveable by just reconnecting. I'm not sure what, but there was a reason why my girlfriend and I didn't play it. We played RCT2 a bunch, with one person watching and the other playing, and OpenRCT2 with multiplayer seemed epic, but there was something annoying in multiplayer, I just don't remember what.
By now, it might have improved again. I remember the development going really fast before. And in singleplayer, I don't think there were any bugs that prevented me from playing.
Give it a spin if you were (or still are) into the original Rollercoaster Tycoon!