Very true. Isaac Newton considered himself to be a deeply religious theologian first and foremost; the work he did in physics and mathematics was a side hobby and many times was pursued to prove some religious point. As it happens, Newton's theological writings make him look raving mad even to the most ardent supporters of Christianity, hence why he is generally only remembered for his work in mathematics and physics.
An oft observed trait of brilliant scientists is that they do most of their best work when they are very young and rapidly slip into mediocrity and nuttiness with age. Einstein is an example of someone who was brilliant in his twenties but essentially became a crank with some dubious beliefs and ideas when he was in his middle age. It is mistaken to assume that brilliant people are always right or even rational.
Of course, Einstein's problems with QM were primarily philosophical. He didn't like philosophical consequences. Of course, he never disagreed that QM gave the right answers, he just thought it incomplete. From today's perspective, it seems that QM will always be incomplete, using Einstein's version of completeness.