From an instructional point of view, Fundamentals is definitely the hardest course that I teach. The concept behind fundamentals is to start the process of getting divers to dive within the GUE standards. Students can come in with vast ranges of capability or experience, and the challenge is to help them improve their diving. It is generally also the first time those divers ever get detailed, honest feedback about their capacity and knowledge. Therefore people work very hard to achieve their goals during the class, and that is tiring. Level 1 classes we start from a known place - Fundamentals Tech Pass, and can build from there. That's not to say the classes are not challenging (they definitely are), or tiring (definitely again), but divers coming in to those have a good understanding of how GUE training works, and will have practiced their fundamental skills a lot. If someone comes to a level 1 class without that practice however, they are going to have a miserable time, and probably drop out on or around day 3.
HTH
John