I've used both high end mechanical pianos and high end electronic pianos, and the biggest difference I've noticed is that the electronic pianos only had stereo

I've used both high end mechanical pianos and high end electronic pianos, and the biggest difference I've noticed is that the electronic pianos only had stereo speakers. Mechanical pianos have spatially separate sound generators for each note, so you get a much better surround-sound effect as you move your head. However, there's no theoretical reason this couldn't be simulated with a sufficiently large number of loudspeakers.