All of my customers had basically no problems, including one with a very old unix micro (with serial terminals) that was obsolete in the 90s, but still in use in 1999 and after. Only issue I have seen was with Novell netware 3.11, that began showing years as "19100" and so on. That system kept working until 19103, when it was replaced by a Linux-based server. And still the application software on it went on naming some files (that had a date-based name) with "19100" and so on. It's still in use today, in 19119.