Eclipse's uses XML to declare UI, behavior, expressions. Completely baffling. Terrible docs. Extremely difficult to debug. I was reduced to trial and error (after copying working examples I found).
I've used Eclipse since the early alphas. Its muscle memory.
Scouting the plugin dev stuff of IntelliJ, it doesn't appear to suck so mightily. I've forced myself to start using IntelliJ.
I miss LISP so much I could cry. Even so, I remain wary of metaprogramming. And I very much want to do plugin dev, vs modifying the runtime, for the separation of concerns. I've had to customize all-in-one runtimes and borking the whole runtime with a bad override is not fun to debug.
I've not done emacs plugins, so can't comment or compare.