I think it’s a pretty complicated question, actually.
Of course, there are all these (part objective, part subjective) parameters that make a sound “good” in a musically useful way. I think part of the problem with the violin family of instruments, is that they have a pretty harsh and nasty “natural” voice, and even though a good player has a lot of control over the tone, the instrument itself still needs to strike a delicate balance of resonances to sound pleasant, even when played really well. So, it’s about the worst family of instruments to model! Ever so slightly off, and it just sounds terrible, or at best, weird. I believe this is true to some extent for all instrument one would refer to as expressive and interesting: They are balancing right on the edge of sounding horrible, like a human voice that is just on the verge of breaking up.
Another side of this is the “uncanny valley” phenomenon. I think the most obvious example of this would be human speech vs voice synthesis. “Retro style” voice synthesis, though perfectly intelligible and also musically useful, just sounds weird and alien. But, when it’s realistic enough that you can’t tell for sure if it’s a bot or a human, you start listening for other things, and unless it’s extremely realistic, it will probably come across as a creepily neurotic person, and at that point, it doesn’t really matter if the tone of the voice is however pleasant.
In short, I think it needs to sound either pleasant and musical (like more traditional synth sounds), OR both pleasant and incredibly realistic, or it will just not work. It’s basically a different flavor of the same problem we have with sample libraries: Not quite there yet, but sometimes close enough to fool most listeners when used in context, and avoiding the problem areas.
So, it’s going to be yet another while before we can fool a professional violinist with a virtual rendering of the Paganini Caprices.
And, we’re still going to need a professional… player of weird MIDI controllers, to pull that off. These are, by all means, real instruments in that regard.