Archy,
I did not find the 2391-20 difficult, but, I was paying myself and doing it evenings in my own time.
I found the maths easy as it was much more basic than the engineering degree level stuff I was used to.
However, even though we had a very good tutor, there was at least a 50% drop out/failure rate.
Almost all of these were experienced sparks, those who got through, you are correct, were the ones who were keen and applied themselves, came to the lectures and asked questions.
The feedback I have heard from the "industry" is that it is not considered easy by practising sparks, as there is very little practical work in the course, it is all book work, calculations, design, research, drawings etc.
That is unless you don't have 2391-10, in which case you also have to do the I&T practical.