What features does a "proper IDE" offer that, at least Neovim, does not or cannot provide?
Class hierarchy visualization, database schema mappings, the latest & greatest in NLP & code completion via plugins. That "just works."
Ultimately, a sufficient amount of configuration adds all those things to Vim/Emacs. Ditto VS Code on a smaller scale.
The issue is how much time.
I use all the obscure features of IDEs. Navigate to next issue, bookmarks, you name it, I've got it, for large codes.
Implementing... all... of the features Intellij/Rider provides out of the box for JVM/React stack in Vim is... an undertaking.
Don't get me wrong. I love SpaceVim, and if I worked in a domain like firmware, I'd be far more comfortable with it.
Maintaining a Vim config that's integrates every tool I need for JVM and React (and machine learning, and all the other domains) is impossible.
I think it is totally possible to be (even very) productive in VIM. I do think, though, that a person who is productive in VIM would be more productive if he used a proper IDE.
This is my experience after I had been using VIM for about 10 years and switched to an IntelliJ IDE (and actually learned it)
(This, of course, assumes that there is a proper IDE for your language)