The immediate function on Korg's is just that... immediate. So you MUST hit the fill button with millisecond accuracy to not destroy the groove. Few can even play the keyboard with that degree of precision, yet alone leap off the keyboard, rush over to a fill button, hit it precisely on the beat and rush back to playing the next chord...

But perhaps those that report timing issues on some Korg styles have those styles set to 'immediate' and don't know it?