Invoke dynamic means less of a need for C-Extensions, which is probably one of the largest sources of pain for JRuby compatibility. The JVM is better at running plain old ruby code now, so there's less need to go outside of it with C or Java.
Well, you don't need extensions, but lots of library interfaces still use them because they haven't been updated to FFI. So sometimes you can't use the code you want. The good news is that it's usually pretty painless to whip up your own FFI binding.
We've been very lucky to have a lot of ingenious JRuby users help port some of those extensions or produce API-compatible wrappers around equivalent Java libs. There's still C exts we don't have equivalent JRuby exts for, but things are much better than they used to be.
That's definitely our hope. JRuby has come further than any implementation in making Ruby fast enough to replace C or Java, and if it's possible to write more in Ruby then all implementations will benefit. C extensions are the number one thing holding the C/++ implementations back.
