You might also be interested in vundo: https://elpa.gnu.org/packages/vundo.html. It is a newer alternative that is to my knowledge more robust, but depends on Emacs 28.
Could you elaborate on "more robust"? I use undo tree currently and the readme suggests it has less features- does undo tree have problems that vundo solves? I don't mind undo tree needing to be enabled all the time or replacing undo commands.
I don't recall the implementation details, but I have heard of undo-tree landing in invalid states that can break the history. It might just be that vundo is newer, but I haven't heard of these kinds of stories for that package.
In my eyes it is the more idiomatic implementation.
I have not used vundo, but it seems to be just a visualizer for the buffer undo list, whereas undo-tree takes over the undo system, replacing not just the commands but the data structure (https://github.com/apchamberlain/undo-tree.el/blob/help-in-v...). In other words, I think there's simply less surface area for vundo to have or cause problems.
I’ve heard the same about undo-tree, was wondering if that was what you were referring to. I haven’t experienced it myself but maybe I’ll check out vundo before I do..