A lot of things like older versions of Gradle (which at some point was a blocker to move to Java 9, but not now), Groovy (also fixed, but still has some warnings), which we use heavily for tests, several reflection-heavy frameworks (think dependency injection, annotation-processors etc), which to this day require some ugly hacks like --patch-module to "open" packages, JUnit tests (which also need the same hacks to work), even the location of resources loaded with class.getResource() had to be modified when modularizing a jar, so that resources go into the proper packages that are visible to the loader...

You seriously didn't suffer any of these?

No I didn’t experience any issues like that myself.