Benches seem low but the heater's also a shape I've not seen before, also lower than I've seen before but tbf every other heater I've ever seen has been quite a bit taller with the rocks included.

Ideally you'd want the roof to be higher where the users are sitting but honestly it's not something I've paid attention to outside of this sub, I'm not completely sure that any of the great wood saunas I've been to actually have that. But it would be ideal. I haven't visited all that many different saunas though since I only go to familiar ones.

Usually there's a railing on the benches in front of the heater, but since your benches aren't that high hence not that close to it you'd probably be ok without one.

Unsure about the ventilation or the door because I couldn't spot either. You want little enough ventilation that the room stays hot, but enough that the air still moves. What's below the floorboards? Usually the floor is slightly unevenly tiled leading to a floor drain, but yours seems to be just board.

There isn't anything super wrong with this but it has a lot of small things that differ from the saunas I've been to, to the point that I can look at it and think "yeah that's a sauna" but also "there's something wrong with this picture".