Similarly, I have unlimited persistent per-file undo turned on in Neovim. I can open any file I've edited previously and walk through the full history of how it got there. With Undotree [0], I can even navigate branching paths in development. I don't know how people live without this.
What are your undo settings? I set undofile and undodir, but not sure if it's unlimited.
One issue I have is if nvim is closed and the file is touched by some outside process (say git pull) it clobbers the history. Do you know if there's a fix to that?
[0] https://github.com/mbbill/undotree