I think that's the most important differentiator evil has.

Embedding vim-style editing in an IDE or less extensible editor might get you 90% of what you want from Vim, but that 10% can be a dealbreaker depending on your workflow. Without easy extensibility, you might not be able to get that last 10% at all.

I'd say evil starts out a little better, maybe 95% out of the box, but now you have all the power of emacs to make up that last 5% and then start to go even further if you're not content then.