"I imagine it's better for many of the same reasons that a mountain bike is better than this:"

I don't think the comparison works. It seems to me that being on 4 tiny wheels as with land roller is more stable than 2 tiny wheels as with the mini bike. The mini bike has a gearing system but I wonder if you still would not be better on hills with the skates. It is true that with the mini bike you are sitting which might be important to some users.

All in all though I think if you are fit and want to travel on small wheels, land roller is better than a mini bike.