For myself I had no issues with JRemote up to MC 18.0.206. But somewhere between 206 and 212 I started to have random issues with JRemote not working (stop, forward, etc) would not work. I would get stoppages of songs for 5-30 seconds, then they would start again. And then randomly stop and start.
I also tried "My River" and it experienced similar issues, just not as often. Sometimes it would get so bad that only stopping JRiver and starting it again fixed the issue - JRiver would not respond.
I decided to upgrade to MC19 and even ran up to 19.0.34 to see if that would correct the issue. Nope. So this weekend I went back to MC 18.0.206 and I had ZERO issues using JRemote with JRiver. I even used "My River" and it too had no issues.
BTW, If I run 19.0.34 right from JRiver I have no issues. So there is something with JRemote and JRiver that has changed since 18.0.206. Maybe some functions changed (variable passing) and information being exchanged is wrong between the two.