They have 12 hospitals participating right now [2], which is in line with the ~10 systems that interoperability plays typically get on board. Apple has been moving on health records for a while, with their acquisition of Gliimpse [3] a couple years ago a clear indication, and I had hoped that Apple would have been able to get more systems participating before they made an announcement. 12 systems is a far cry from the 5,500 hospitals [4] and 230,000 practices [5] in the US.
In order to make medical records really useful for people, they needs to have all of their records. That means records from whatever system they've been seen at, and all of the information in those records. Every purely electronic approach to aggregating records that I've seen doesn't get doctors notes, procedure reports, pathology reports, radiology reports or medical imaging (x-rays, MRIs, CTs, etc). Sadly, it looks like Apple's attempt also won't address those issues.
Collecting medical records is an extremely hard problem. Technical integration is not only expensive and annoying (see many other comments about the shittiness of HL7, which FHIR is the latest, least-bad iteration of), but health systems don't have strong incentives to push interoperability. Health systems have way bigger IT problems facing them than aligning with interoperability standards (e.g., update the failing, 10-year-old software in the pediatric ICU), releasing records automatically could open them up to a lot ill-defined of legal risk (is a health system liable for what happens to information it releases to third parties?), and interoperability is kinda against business incentives (more interoperability => easier to lose patients). It doesn't look like Apple is taking a fundamentally different approach than Google Health did to any of these incentive problems (if we build, it they will come).
disclosure: I'm one of the founders of PicnicHealth (YC S14), and follow these things pretty closely. If you want to chat about the space, feel free to email me: troy{at}picnichealth{dot}com
[1]: http://argonautwiki.hl7.org/index.php?title=Main_Page
[2]: https://www.apple.com/newsroom/2018/01/apple-announces-effor...
[3]: https://techcrunch.com/2016/08/22/apple-acquired-gliimpse-a-...
[4]: https://www.aha.org/statistics/fast-facts-us-hospitals
[5]: https://en.wikipedia.org/wiki/Group_medical_practice_in_the_...