Let me share my little knowledge I have concerning your issue.
First, most MCU silicon vendors do not readily publish failure statistics figures like MTBF.
You might get them on request.
Second, you will need to take the constraints of you application into account.
Meaning, what are the operating conditions ( device loads, temperature, temperature cycles, EMI).
But according to my limited experience, most MCUs operate under very moderate conditions, and are not very prone to fail.
Besides of exceeding maximum rating values due to poor/cheap design (neglecting EMI protection), the Flash is often the first to give up. Either through wear by frequent erase/program cycles, or simply loss of content during long operation (decades).
