Of course one disadvantage of mechanical instruments is that the performance can sound mechanical! For example the simplest machines do not provide for any variation in the volume of sound, and similarly the rhythmic pulse is often completely invariable, giving a remorseless "sewing-machine" effect. Ingenious methods evolved for overcoming these shortcomings; for example by the beginning of the 20th Century Duo-Art piano rolls were able to reproduce quite subtle variations of rythm and volume, so that piano rolls made by pianists of the time such as Paderewski give a real improession of what their playing sounded like - certainly better than the flat scratchy sound recordings of the time.
Computers provide really good answers to the problems of mechanical music-making. One can create and control musical sounds by writing a MIDI file, but the process is complicated and too inhibiting for most musicians to create really musical performances. The SIBELIUS file is much more friendly since you can write directly in familiar musical notation, and use familar expression marks to humanise the performance.