Thanks a lot for trying out Planet Hex and giving feedback!
One of my inspirations on this project was also to get a better understanding how physical planetary phenomena and properties affect each other. It's fascinating indeed!
Yes the simulation happens on the hex plane. Each hex has their own properties. The x boundaries wrap around, however i think at least rivers might have some bugs wrapping around. The y axis does not wrap around at all.
Yeah sphere would obviously be more realistic but eventually I would like to create a 4X strategy game and I think simulating things on a sphere might be overkill and too big scope for me right now.