I'm not a training director or have enough background process info so I don't know if every site that has unmatched spots from Phase 1 automatically participates in Phase 2 (or is required to regardless of site preference) or if each program has to specifically affirm their preference or withdraw from further participation.
If it's the former, that's less than ideal and would put unorganized/disinterested programs in a position to receive applications that take a lot of time which may not be considered.
If it's the latter, then these sites are likely still reviewing applications in good faith and with due diligence but potentially not extending many/any interview offers based on how their application review process goes. I'm sure there are exceptions (but hopefully not too many since I've found people interested in training to be some of my favorite professionals in our field but that's my bias).
The best case scenario is that a site finds applications from people who could have matched with the site in Phase 1 had they applied/ranked the program, do a handful of interviews, and match with one of those people.
The worst case scenario is that a site only receives applications from interns that they don't believe would be a good fit for their training program and don't offer any interviews or submit any Phase 2 ranks, which sucks for applicants.
As far as transparency goes, there will always be some inherent lack of transparency (why somebody received an interview versus somebody else who didn't, why an applicant ranked site A but not site B, etc). I agree that more transparency such as if a site is no longer considering Phase 2 interviews, that should be made public to inform people and potentially save Phase 2 application time.
