Typically about 5% of medical school graduates don't get matched, although some of those do get in later.

https://www.ama-assn.org/residents-students/match/if-you-re-...

As for overseas medical schools, the National Committee on Foreign Medical Education and Accreditation is supposed to ensure they meet the same standards as domestic schools. I haven't seen any real evidence that they're sketchy.

https://sites.ed.gov/ncfmea/