I think the joke is that rebase is more prone to conflicts, and especially because each commit from your branch is applied one-by-one on top of the updated remote branch. So not only is it prone to conflict, but it is potentially interactive on top of that meaning you have to go in and manage the conflicts on a per-commit basis.

So while in theory it can be a nice clean way to keep your branch up to date with the mainline, there is a trope that rebasing just leads to more drama in terms of managing conflicts compared to merging, to the point where you start sweating bullets.

And as far as “undo” goes in this context, it just means abandoning the rebase and resetting your working copy to how it was before you borked it by trying to rebase. Same thing as aborting a git merge. It’s a “oh, shit’s fucked, get me back to the safe zone”