Since you mention Emacs, I'd like to point to one thing that really only Emacs has, and nothing else: If you implement your "app" with Elisp, it is truly UI independant. It will work in a terminal, and with a GUI. To my knowledge, there is no such thing elsewhere. I used to write a lot of Elisp, roughly 20 years ago. And I did it all in a terminal, because this is what I can use. And users of my code could use it in their GUI environment, without even knowing that I was coming from a plain text-mode background. Again, this is very special, and IMO there is no other "platform" around which can do that.
I think Neovim is more flexible -- a plugin you write for neovim can run in the terminal, in any Neovim GUI or in another application (like VSCode) that can embed Neovim.
I tried to use Neovim several times. However, it appears to not be very friendly for my usecase, which is using it with a Braille display. A lot of Neovim UI elements only use background/foreground color to indicate highlighted positions, instead of making use of the cursor. I couldn't even use the intial plugin menu. Emacs is much better in thsi regard.
