Hard to argue that Einstein is not overrated when he is literally the archetype of 'genius scientist' for most people in the world. But in his annus mirabilis Einstein literally made world-tier steps forward in 3 huge areas of physics simultaneously. Any of these in isolation would have ensured his immortality. This is either the single most important short productive period of scientific work of anyone, ever, or holds that accolade in tie with Newton's plague year.

Of course, his later years did not live up to this stellar record - he both made mistakes and was less productive. But how could they?

Joseph Heller was challenged by a journalist in later years that "he hadn't written anything as good as Catch-22 recently". He replied "No. But who has?"

Einstein "got" Quantum Mechanics better than most physicists of his era.

Einstein was one of the pioneers in Quantum Mechanics, making foundational contributions to the understanding of the quantum nature of light (the photoelectric effect and stimulated emission). Even in his criticism of certain elements of Quantum Mechanics, he was incredibly perceptive. The Einstein-Podolsky-Rosen Paradox is a really difficult problem, which gets to the heart of the apparent strangeness of Quantum Mechanics.[0]

0. https://en.wikipedia.org/wiki/Einstein%E2%80%93Podolsky%E2%8...

I'm currently reading through a book that pretty thoroughly eviscerates that last point as a myth. Part of the argument is that he was resistant to efforts to bring classical concepts into the new domain, thinking that something entirely new may be needed to explain reality. He also spent a lot of time (naturally) trying to square it with GR, something we're still working on today.

He kept up with the math and the experiments, but took a contrary view on the interpretation. As an interesting side note, the Copenhagen interpretation which was emerging around that time is now disfavored by the wider physics community (except as a calculation tool).