I’m sure I’m not the only one, but quantum mechanics was something of a “brick wall” I hit in my study of physics.
Up until that point, I was comfortable with physics, even if I didn’t understand every nuance. But I could never get my head around QM.
Those are the students that go on to contribute to QM. The students that think they understand it cause all the trouble.
Likewise...
The reason is it becomes almost purely mathematical.
A particle is no longer a “spherical object” like a ball or a planet; it is a mathematical equation.