Individual or group guitar lessons?
A subjective question, I always say that each student is a world and that the teacher cannot use the same pedagogy with one student as with another.
Group guitar lessons
I think it does not matter so much if the classes are individual or group, if the teacher uses curricular adaptations for those students who need it, that is, there are students with ease to learn and others with difficulty, and it is these two situations, which The teacher has to discern to meet his needs and that is what I try to do in my flamenco guitar academy.
Within the group guitar lessons, the guitar teacher has to give a syllabus and help both those who have a hard time, and those who need more material to learn because they have more capacity for assimilation.
Individual guitar lessons:
I come from a family of flamenco guitarists and therefore my first teacher was my father, I must admit that it has its advantages, but I also see that not having social relationships with other students can be inconvenient. Normally, when there are several students learning, I see that there is a healthy competition that helps the student improve. Also, seeing the mistakes that your classmates make helps you better assimilate learning and not make the mistakes that others make.
In short, in my time as a student I had both options and I must say that for me the most fun were the group classes, although the individuals were more direct, I think that the human being needs to have fun and have experiences for a good learning.