Ah.... the joys and wisdom of personal taste.
Regardless of whose board you pick - Yammy, Korg, Roland or even GEM - the pianos that come on the stock boards are all OK. I said OK.... not "great", because great boils down to personal preference...

Do you want a piano sounds sampled from a $60,000 Bosendorfer grand or an $8,000 Yammy upright. I've heard some of the latter that I thought were pretty impressive. OK... choice made then ? Not enuf ambience, reverb, delay, chorus and so on. So add until the mix is how you want it and save it to another program/location!! Even replace the original if you want !! Anyways... Ta-Da !! Done.... your own personal piano sound.

If the pianos sounds on the Korg were that bad... do you think they wouldn't have figured that out yet and fixed them already - at least on the flagship synths ??

My point is that all synths have basic pianos, whether you like them or not is up to you. Just 'cus you don't like it, doesn't make it bad. Of course, there's no beating personal preference, is there !!

Regards,
Steve