For medicine, there were so many uworld questions that I didn't even finish uworld IM in time for the shelf. Wouldn't sweat it if you can only do uworld (or not even finish it) for the medicine shelf. Where I really wish I had amboss was in surgery, because I finished uworld early and had no other source of high quality questions. That was the hardest shelf for me.
A little bit on the pros and cons of uworld vs. amboss: Uworld: more straightforward, more realistic questions, vignettes read more similar to the actual thing IMO.
Amboss: these are HARD and oftentimes intentionally tricky, but they give you confidence for the real deal. overall, less realistic IMO. it's also kind of like wikipedia where you can scan over unfamiliar terms in an explanation and it has a brief description, which i found nice. the amboss library was also a good feature if you want a little more info but not nearly as much as up-to-date, etc.