If your used to a particular keyboard OS then all the competition seem much harder to use and to get your head around, however if you look on the various forums you will find that newer users into arrangers always find Korg the hardest & Yamaha the easiest to learn. (The others fall in-between)

The reason for this I suspect is that on the whole the Yamaha has few feature and editing capabilities compared to the Korg, which has a massive amount and thus causes confusion to new users. (Even easy mode is more comprehensive than a lot of Yamaha Arrangers)

Bill