I had a close look at RxJava and decided I wasn't smart enough to understand it. I intentionally didn't tell my colleagues about it, they would have loved it and I would have been in big trouble.
Promises are all over the place too. I understand why someone thought this was a good idea ("I'm doing the scalable thing!"), but in the end it appears completely unnecessary (e.g. doing a blocking get call in all uses).
The problem with Java isn't Java. The problem with Java is that it lets so many people hold it wrong, and the Java ecosystem encourages it.
I've been coding in C recently and really enjoy the limitations. Usually there's only one way to do something.