in a codebase with tens of thousands of lines of typed code, I see maybe a few new type errors with a new mypy release. They've always been fixable within a few minutes.
I think mypy has some problems, but this isn't one of the bigger ones for me.
I don't think it's a "problem" with mypy, I just think it's likely the cause of a lot of the programs that the authors think don't type check.
Though I will say, while most have been fixable in a few minutes, some have been a real chore to fix. Sometimes an innocuous looking error balloons into several hours of reconciling obscure type system behavior errors once you start fixing it. Regardless, it's a small price to pay for proper type checking in Python. I've more than made up the lost time in detecting bugs before they ship.
I think mypy has some problems, but this isn't one of the bigger ones for me.