But you still have to share the undodir for both vim and neovim right? If your undo data was that precious why would you gamble the interoperability? You could just have them in different folders and it would be fine.

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.