Has anyone had quality concerns when this may not have happened?
I have seen quite a few quality concerns associated with medical translations during my career as a translator/editor.

In my opinion, one reason behind this is the great variety of medical specializations. No translator can know them all. Some translators however assume that by achieving a certain degree of subject matter expertise in one or several specializations, they can easily translate any other field, and when they do so, they fail to do a good job. Other reasons are more general such as a translator being lured by an attractive price, while they do not have the appropriate knowledge or skills to do the translation.

Do you know for sure that the correct resources are being used on your projects
This is indeed an issue in the translation industry where a job can be pushed through so many outsourcing levels in pursuit of the lowest price that the ultimate $0.01 translator would not be qualified even to write a high school essay in the target language, let alone medical translation.

I think one way for a client or agency to be sure about a vendor is to carefully select and get to know your translator very well, then establish trusted long-term relations. This way, you will be confident that the translator is committed to the high level of quality you expect and will not outsource your work to someone unqualified to do it. Otherwise, you are at a great risk of receiving “anonymous” translations made by potentially unqualified people.