From personal experience, when you want to get a solo instrument to sound good (as in realistic good) - specifically strings - it's incredibly important to do more than just articulation and CCs. It's about the timings, the swells of the expression, the up-bows, down-bows, the human qualities that a string player would inject. As such, doing that in computer facing a screen of options and hundreds of bad choices that you might make obviously make that seem pretty tough. I still tend to avoid solo string instruments excluding violin because of this kind of difficulty.
There's a researcher in the music department of my university who is currently working on actually physically modelling string instruments specifically so that they can create a more realistic human feel to a sound, but the basic gist of everything is that an actual performer does a lot of things subconsciously and as such, modelling these is very very difficult.