I've done this a few times. Most are due to reflection limitations with the newodule system.

They can be worked around by adding exceptions to removal of reflection metadata largely. Kinda a PITA but not too hard.

Active libraries have pretty good Java 11 support these days though. It's really improved a lot on the past year or two

To Java 9 yes, mostly due to third party dependencies and the move to modules.

To Java 11 you might have additional hiccups when using JavaFX, or the JEE libraries that were also being bundled with the Java SE.

Which wasn't our case.

Donald Smith the (one of?) product manager of Java at Oracle did a talk for the London Java Community[1] recently. He briefly mentioned that getting onto Java 9 was the hard part, 10->11->12 are significantly easier due to the far smaller number of changes in the release.

The meetup was recorded[2]. I can't tell you how far through he mentions it, but its a good talk to watch as he goes into some of their rationale for their approach.

[1] https://www.meetup.com/Londonjavacommunity/events/258987722/ [2] https://www.youtube.com/watch?v=RWpo92OBaNA