Flash memory often have limited write-erase cycles.
The datasheet has that figure.
There are mcus out there (modern ones) that have a flash endurance cycle of 100 times (yes, that's 100 times). I thought that was a typo but it wasn't fixed in subsequent versions of the datasheet.
Seriously, though, even 100 programming cycles is more than 99.999% of applications need. I guess hobbiests who program to test every minor change they make would not be one of them. The big issue is that the onboard EEPROM (if provided) might have the same specs which would render it unusable for many applications.