[rr developer here]

Undo has cool features like Live Recording that we don't have in rr. They don't need access to the hardware PMU which is a big advantage in some situations. They can handle accesses to shared memory in cases where rr can't. https://undo.io/resources/undo-vs-rr/ is a good resource.

If you don't have access to the hardware PMU then you can try https://github.com/sidkshatriya/rr.soft (which is a modification of the rr debugger).

It may not be commercial quality but its open source and free :)

[I built rr.soft]