Jonathan Aigner, with his usual crisp pungency, argues that the guitar is an inadequate instrument to support voices in public worship. I agree with him for the reasons he states, but also because one usually plays a guitar on different beats than one sings. Further, it is next to impossible to pick vocal parts out of guitar chords unless you know your part by heart already.
Now, at the risk of being called a hypocrite, I will disclose that we occasionally meet as a church at our house for an evening service; and for those occasions I will ask our resident musician to bring his guitar. But I would never ask him to lead us in a complex or unfamiliar hymn for the very reasons Aigner gives and the ones listed above. The guitar may be fine for informal gatherings like potlucks at the parsonage, but it’s not adequate for Sundays.