Well, I don't get what the alternative would be. As a student, I expect the exam material to reflect the notes, readings, and lectures. If the exam doesn't do this, then the notes, readings, and lectures are useless, from my vantage point.
I haven't started teaching yet, but it would be unfair of me to ask students details we've not covered in class or in the material, because everything I'll cover is worth covering under the time constraints.
I should note that the questions I would ask on an exam depend on the audience. If I'm teaching a masters level class on causal inference, I will only ask why, for example, the convex hull constraint is believed to be useful. I won't, for example, teach them about changing the constraints to allow for negative weights, allowing for intercepts, or taking away the adding to one constraint.
Why? Well because I honestly would expect very few masters students to even try to do well on a question like that. If I've only covered the classic method, then that's all they'll need to know about.
And even if it were a PHD course on synthetic controls, I would still at least talk about these different penalties and approaches to constraining the weights. I would demand they have knowledge of the basic setup, and then be able to talk richly about why changing these details might change our results. Why? Well, they're PHD students, and this hypothetical course would only be an elective, so if you're here in the class, I presume you want to know deeply about this method in econometrics.
You've already mentioned that your students are average (which is fine!!!). Why even bother to ask them things you know they won't do well on, if the median student in the class is average? Part of being a teacher is teaching to your audience, and putting things in a way they will understand, and testing them in a tough, but FAIR manner.