if you ever listen to a guitarist play its the fret noises, slides dead notes and bends that make a guitar sound authentically like a guitar. Replicating that from scratch on a keyboard is no easy task regardless as to how many samples are in the sound or the size of the sample. Trying to trigger a slide or a muted sound or a scrape as intuitively as a guitar player would is tough. Thats why the styles that utilise these techniques in the yamaha/korg are so great and really difficult to get similar results when you have to try and programme all those elements cohesively and convincingly. It is easier to do with monophonic sounds like saxes and basses. The best Bass emulation i have heard so far is this one http://www.spectrasonics.net/news/2010/artist-videos-george-duke-greg-phillinganes/

Guitars playing chords is a lot harder but i guess with enough tiome and practise anything is doable.