Extensibility via plugins is quite a bit different from extending the editor as you use it. Emacs admits the latter, and useful functionality can be added in a page or less of Emacs Lisp.
I tried writing a plugin for Visual Studio Code. It made me want to throw furniture.
That is true. IDE plugins are a nightmare to write and VSCode is by far the worst to write. Still, JetBrains is remarkably extensible and mature, I was able to integrate things very deeply in the editor. The downside is writing in Swing which is pretty long in the tooth by now.
I tried writing a plugin for Visual Studio Code. It made me want to throw furniture.