That's not quite true for original Vim. The developer experience for writing plugins is incomparable; VimL lacks good ways of abstracting and composing behavior, while Emacs Lisp gives you just about everything a modern language should, including sophisticated object system with multimethods and multiple inheritance. The built-in debugger for Elisp is not on the level of JetBrains IDEs, but it provides all the typical functionalities and is GUI-driven, in contrast to Python's pdb or Ruby pry.
NeoVim is an entirely different beast, and I heard good things about its way of handling plugin development.
It's just a matter of having the will to do it and putting in the time.
I say this as a huge fan of both editors. They are both so powerful they can do just about anything.