I agree with everyone. The only thing that I might add is that even the best instructors will miss some things (poor form, bad technique, etc.) in a large group setting. All the cueing could be spot on, the instructors knowledge and education could be first class; however when the ratio is 20+ participants to 1 instructor, something is bound to slip by every now and then.
I like to keep my classes to 10 or fewer because of this. If I go higher than that, I like to bring in a second instructor to improve the ratio. Sure I make less money this way, but I feel better knowing my clients are getting the most attention possible in a group setting.