The main challenge I see with PBL is using the approach in large groups of students. If you have a class of 30 students and broke them into 5 groups of 6 students, it would be impossible for the professor to give his/her full attention to all of the groups.
A survey was conducted by Long and Qin (2014) at a medical college in China, where a group of 36 students were divided into six groups. The professor reported that PBL was effective in the large group and student feedback was positive. However, there were some challenges noted.
1. For a professor to be successful with PBL they need previous PBL training/experience.
2. It is challenging to create identically difficult problems for the groups.
3. The success of a student group is dependent on the individual characteristics of the students.
4. There was no significant difference in student performance between PBL taught students and traditionally taught students.
Frambach, J. M., Driessen, E. W., Chan, L., & van der Vleuten, C. M. (2012). Rethinking the globalisation of problem-based learning: how culture challenges self-directed learning. Medical Education, 46(8), 738-747.