I'm not sure I've quite understood your doubt correctly, so I'll do my best and I hope it will help you.
I would say that the most common issues you can find when you have a specific purpose, are
a) Classes are not always directed to one student's needs, they are normally general, covering general not specific topics.
b) Keep realistic standards (whether for the content and for the student´s level)
c) What you expect is not always what the group expects (They may pay more attention to topics this student is not very fond of going over and they may eliminate or diminish others the student considers essential.
For Teachers may seem easier to focus and deliver a class for specific purposes because he/she is indeed "in control", however
a) Students may not achieve as well or as much as the Teacher expects or needs.
b) Finding meaningful, focused, and appealing activities and material can be almost impossible or the teacher has to prepare it, so it makes it really tough.
c) Some structures or topics are hard to understand and harder to teach, therefore, if a group is not strong enough, it would be truly difficult to achieve the stated goals.
d) If a teacher lacks of support (from superiors or school or else), this can be decisive.
Well, I'm sorry but I've got a lot of work, so I hope I was able to help you at least with a minimum part Mónica