The Technics OS was probably the simplest there was - not much to it. You could pretty much learn the entire OS in a couple nights, just sitting at the keyboard with the instruction manual.
The Roland and Yamaha were about equal, technically. Both had their idiosyncrasies, but generally speaking, the most difficult registration system to learn was Yamaha. That took me close to a week to get it right and become second nature. I thought Korg's OS was about as easy as the Technics, with a few exceptions. Of course, I'm sure they've all changed since my days with those antiquated keyboards.
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)