Korg is great in so many ways, that is no question about it.

My three major grips with it (Pa900) are:
1. Very thin snare sounds in all drums kits pretty much.
2. The old file system management.
3. The fill-in issue, whereby with may styles the fills have a sort of hiccup I called it, if you are not PERFECT in timing, the flow of music is broken and the "hiccup" is very audible. Have a look at this video https://www.youtube.com/watch?v=Tputto6sCN4 and you'll understand. At 0:57 and 4:08 when fills are pressed. As you'll see many fills sound just fine, while others do not. This problem is present in my Pa50 and even in Pa3x...

Other than that, Korg is great indeed!