Undo co-founder here. rr is indeed awesome. If it works for your use-case, you should use it!

Undo is mostly used by companies whose world is complex enough that rr doesn't work for them, and they understand how powerful time travel debugging is.

There has now been a LOT of engineering invested by a lot of very smart people into Undo, so it does also have a lot of polish and nice features.

But honestly, if rr is working for you, that's great. I'm just glad you're not doing printf debugging the whole time :)