Well, Ray cared about the sounds being as good (Athentic) as possible and the Kurzweils still have that going for them.
Not so much with others....
Even today, I can go play a TOTL arranger and fins many sounds not correctly done.
I had a PA2XPRO and that is one main reason I sold it. The sounds (many) had problems,

There are many different sound design issues you hear.....very complicated to cover here.

IF you play only fast, dance type music then it is not near as much of a big deal...BUT if you like symphonic, musical theatre, standards, movies music etc...then the sounds need to be great quality...as there is no singing, no loud drum tracks, no distortion guitars etc. to cover up the poor sound quality of the instrument.

One example I can remember on the Korg was the Sax's.....if you played up/down the keyboard you could hear the problems with the samples. Another example is on violins...the upper end of the instruments range often has a digital noise instead of the beautiful sound of a quality violin.

All that said...they do have a LOT of really good sounds too.

I think Kurzweil is still the best...and I wish they made an arranger keyboard using those sounds.