There are still plenty of effects in real pianos that are not emulated properly. Two examples: resonances in the other strings of the piano when you strike a string, and striking a key, leaving it half-pressed, and striking again. The piano pedals are also not easy to emulate, I understand, but I don't know the details.
No question about it. There are things I can do on a grand that I can't do on any synth that I've tried. One trick I like to do is bell tones, where you give it just enough sustain pedal to get a little bit of ring and then play semi-staccato. That falls pretty soundly under "extended techniques", of course. In principle, it shouldn't be hard to make a software synth that can emulate that behavior, just by using a volume pedal instead of an on-off switch for the sustain pedal, and writing the software to model the instrument's behavior sufficiently. AFAIK, nobody has done it, though.
For that matter, I have yet to hear one that emulates the extra richness you get from a piano when you push the sustain pedal, but I haven't used any recent piano VIs, so I'd imagine somebody has done it by now, given how trivial it should be to emulate.
Either way, digital simulations of pianos are good enough to be generally usable. That's more than can be said for any digital brass I've heard to date. For example, consider the Garritan family of trombone sounds. Despite the fact that pretty much any normal tenor trombone played by any professional (and most high school and college students, statistically) has an F rotor, none of their trombone sounds go down below an E except the bass trombone stop, which makes them all almost completely and utterly useless for real-world use, where non-bass trombone parts routinely drop into that range.
The bass trombone stop has the range, but unfortunately, it has a very slow attack (which is somewhat realistic for larger bore trombones, mind you). To sound correct, the player should compensate for the slow attack and should play slightly ahead of the beat like a real trombone player does. Unfortunately, it doesn't, and as a result, in fast music, it ends up playing a quarter beat behind the rest of the ensemble, and it sounds like utter crap.
And those are just the problems that are bad enough that even the most tone-deaf person would notice them if you didn't work around them. Compared to that, getting a "mostly good enough" piano sound is easy. :-)