I've used vim since the first time it appeared in Debian repos. I have been told a thousand times that NeoVim is better, more modern and solves many (conveniently never cited) issues. I just ignored it and carried on. Feeling terribly vindicated at this point as it's a feature I use regularly and have no idea that it would be an issue in NeoVim.
I've never used NeoVim because Vim has been good enough.
I used neovim because it felt way faster by default. I think treesitter runs circles around what classic vim is using to highlight syntax. That said, I never used both beyond basics, thus no horror stories either.
[dead]
*vimdicated
If one really needs an undo in today's day and age, use git.
I've used vi and nvi and (now) vim, and tried neovim, but :! is broken and “Conform to POSIX vi” is one of their “Non-goals”.