Music in church is for worship, not performance. Neither virtuosity nor indeed even the pleasure of the hearers is the point. the point is to conform itself to the words of the liturgy or hymn being sung, to support it while not overwhelming it. The piano is a profane instrument, in that its very sound evokes the concert hall and secular pursuits. Quite unlike the organ, it's a percussion instrument, demanding the intrusion of rhythm into the melodic line. Rhythm is a distraction for minds trying to stay focused on prayer.
All "noisy or frivolous" instruments are prohibited for use in the Mass. (Pope Pius X, Motu Proprio Tra le sollecitudini (1903), n. 19. Principle Upheld by the Instruction of the Sacred Congregation of Rites Musicam Sacram (1967), n. 4. The principles in Tra le sollecitudini were generally reaffirmed by Vatican II in Sacrosanctum Concilium (1963), n. 112.)
Specific instruments named by the Popes have included guitars, pianos, drums, cymbals, and tambourines. (Pope Benedict XIV, Encyclical Annus Qui, n. 90; Pope Pius X, Motu Proprio Tra le sollecitudini, n. 19. Principle upheld by Vatican II, Sacrosanctum Concilium (1963), nos. 112, 120, and by the Instruction of the Sacred Congregation of Rites, Musicam Sacram (1967), nos. 4, 63.)
"Bands" also are prohibited, as are all automated forms of music (recordings, automatic instruments, etc). (Pope St. Pius X, Motu Proprio Tra le sollecitudini (1903), n. 20; The Sacred Congregation of Rites, Instruction De Musica Sacra (1958), n. 71.)
So, liturally, no guitars are ever allowed? Does that mean John Michael Talbot's beautiful music for the Mass is not allowed? But what about all the sacred music composed by Mozart? They include instruments other than the organ?