On sample based digital computer stuff:

The quality and amount of used samples also plays an import roll. There are some great VSTs around sound wise, indeed Ample, and Orange Tree.

Most acoustic instruments (and old electric Fender guitars?) have changing overtones (harmonics) and a resonance frequency belonging to the physical built property of the instrument itself. Hard to capture in a digital VST environment. Years ago i was in the luxury situation of owning some early AKAI and Ensoniq samplers due paid sound-tracking jobs. Spent a lot of time trying to capture mandolins and guitars. Tried layering to capture all, but gave it up sort of with a feeling of 'too much work' and 'i'll play it live on tape myself at a later stage', once i figured out the arrangement. Often done with BIAB, Atari days LoL.

These days i sometimes replace a 'real' rhythm guitarist with a VST (easy way out). And the only time i really was impressed of what synths could emulating a lead rock guitar (and sax and blues type harmonica) was with a Yamaha VL1m i owned quite a few years.

Later Steinberg Wizoo came out with their virtual guitarist VSTs, i still use at times for creating rhythm guitar tracks, maybe also out of laziness.
Sounds fairly OK though ...

MIDI produced stuff versus live recorded musicians has still a very big pre; one can change important parts quite easily at a later stage of some production. RTs are amazing, especially some. The trouble with digital tracks (limited to 16bit?) is IMO when you mix them together a sort of muddy-ing curtain moves in covering the whole caboose making all less palatable. Some RT's sound great soloed however.

On 16bit:

Was assisting at some live recording of a classical orchestra once in a church. The 16bit recordings on an AKAI Dr8, properly externally wordclock slaved and leveled, and left as pure as they were, turned out real well sounding on the final CD afterwards. 16bit can work, but i would prefer at least 24 bit if i have to work with sampled stuff in a complex production situation. RT going 24 bit? However it's the same with Toontracks drummer plugs, all 24 bit, but in a mix all those superbly sounding cymbals need extra help and processing to survive in a final mix. -F