https://neovim.io/doc/user/undo/#_5.-undo-persistence

So even putting aside the Vim/Neovim interoperability angle, that contract was broken. Undo files created by old versions of Neovim will be deleted by more recent versions.

The interoperability issue can't be ignored either. Neovim was touted as a drop-in Vim replacement, and that needs to be taken into account when considering how changes affect users.

So many Neovim users have started by reusing their existing vimrc. I'm sure many still have ~/.config/nvim/init.vim symlinked to ~/.vimrc to this very day. With Vim, persistent undo is opt-in. Vim users who cared enough to opt in are highly likely to have explicitly set the path for undo files too. That's because Vim defaults it to the same directory as the file being edited, which clutters the filesystem. As a result, there are many users who shares undo files between Vim and Neovim simply because they reused their vimrc, not because they made the conscious choice to do so.

It seems the author has too high expectations of this feature, or vim is using an incorrect path to store this is they want to make it available more reliably

Not withstanding this incident, I would say that on the whole it has been a good experience with Neovim. (I didn’t actually ever use the persistent undo functionality in vim. Guess because of VCS it’s less needed for my use-case).

So the case is, you really need you undo history and want to try out neovim so you just copy paste you vimrc to the new place and accidentally delete your data? I don’t think it’s that bad personally. It’s bound to happen but it’s not automatic.