I don't recall which physicist it was who said, "You never truly understand Quantum Mechanics. You just get used to it."

As for another subfield in physics, Superstring Theory, I recall a TA who told us when we were still working on our physics B.Sc., "If you believe in superstrings, you may as well believe in black magic. We'll never have any way of verifying theory with any experiment. Certainly not within any of our lifetimes."