But they suffer from “memory exhaustion” rather easily, to the point where even several blackboards on a funky roller system might be insufficient for a one hour lecture developing a complicated proof.

And the garbage collection causes a serious interruption and sometimes misses things.

have parse errors

Your lecturers obviously had much more legible handwriting than some of mine!

have encoding errors

Well, an encoding where P, p, and ρ all occupy the same code point might be considered ill-advised, and one where m, n, r, u, v and w may variously appear distinct or not depending on the display device in use is downright mischievous.

have usability problems

Does blocking half the lecture theatre’s view every time you write up a new formula count as a usability problem?