I would put Roland easiest, then Ketron, then Korg and THEN Yamaha.
I have answered probably a thousand questions on Yamaha OS. (Of course there are lots more people playing them.)
I'll bet I have had to explain how to save a registration a hundred times, and Gary probably has done it 10 times more.
For the uninitiated, you save the registration, then must save the bank, AFTER you tell it what parameters to save and ignore, in two totally different menus.