They are laggy and have lower resolution than a chalk board or a whiteboard.
And what for? What can they do that a whiteboard cannot?
(teachers should write on a board and not use slides, because slides cause teachers to go too fast and to jump over important details.
Slides are entirely appropriate, especially if the teacher is teaching the same class multiple times per day.
I have had Maths professors who were teaching the same class for decades. They would pencil it all out on the blackboard. We copied it by hand. This slowed everything down and we got time to digest the content. First questions could be asked and answered.
And yes, we all had laptops, but we kept them in the backpack.
It was awesome.
(German university ~10 years ago)