For me it's rules about missing timing. Which was a thing for a lot of older cards, but not all of them obviously.

Some cards need to be activated and resolved immediately after specific actions occur otherwise they miss timing and cannot resolve their effect

Like if you don't know what numbers come between 1 and 5 on a Pendulum Scale, then that says more about you and your quality of education than it does about Pendulum as a mechanic in Yugioh