I don't think that deserves to be categorized as a "reliability" problem. I don't think that's technically a reliability problem.

Sounds like it was reliable, with the problem being that it's much much slower than bare metal (a very well known thing), with the microcontroller not having the required processing power to service your task in that context.

Dropping into C for performance critical code is something you sometimes need to do with python, regardless of platform, because it's slowwwwww anywhere it runs. You'll always hit a ceiling, and much much sooner on a microcontroller.