Source is irrelevant
All software has bugs. Sure there's buggy proprietary software out there, but there's far more buggy & crap open source around. Most of that stems from the fact that way too many open source coders get bored once the project is 80% done, and rarely if ever do proper testing anyway (they may think they do, but they don't). People who sell their product for a living have to do some level of proper testing, if only to maintain the level of customer satisfaction required to get paid so they stay in business.
Clearly medical devices have bigger risks, are more likely to lead to court cases when they go wrong, and so require better testing.
The idea that because the code is public it will somehow magically get more and better review and testing is plain daft, even more so in a specialized field where there are very few people with the knowledge required to understand all the corner cases the code must handle.
No way is anyone implanting a FOSS-driven gadget in me, no matter how many self-certified "experts" have "reviewed" the code!