I think according to Feynman the fact that nobody understands QM is the reason why we can't easily teach it to freshman.

In some sense I think modern physics even dropped the goal of understanding. It got replaced by the mantra of "Shut up and calculate."

but certain people do overestimate how impossible it is to explain certain things in a way that can be understood by intelligent laymen (as done e.g. in Feynman's QED).

I also often underrate it. I once tried to teach a first year student in informatics A the principle of recursion. The whole course uses Haskel to make a point of teaching recursion. I don't think why was stupid but the new phenomenological primitive of recursion was really hard to get into her brain. I think I spent 2-3 hours in one-on-one tutoring.

There no way to explain a concept that requires 3 new phenomenological primitives that a layman doesn't have to that layman to make him really understand. You might find substitutions and explain the concept in a way that reduces to primitives he already has, but then you aren't really explaining the full concept.