I tried not to talk about the property itself and focus only on the issue of adding more vendor prefixed elements to the open web and today I want to expand that with my point of view about the value itself: -apple-system.
The theory upon which this new value is introduced is to help designers match the underlaying OS, and so we're supposed to treat it in a way similar to listing all the default OS fonts:
font-family: system, “Roboto”, -apple-system, “Segoe UI”, sans-serif;Except that no designer worth their job would ever do that.
I know very little about fonts, but among those little things that I know is that each font has different metrics, so using such a mix of fonts will be a guarantee that the final result will be a disaster as soon as the user loads the page in an OS different than the one where the page has been designed, the carefully designed UI won't fit correctly because some elements will be bigger/smaller than expected, the font-weight won't give the correct feel, etc... using a fallback font is a kind of "last resort" and when you do that you try to suggest alternatives that would fit properly but you don't have that kind of guarantee when you use fonts designed from different OS.
Furthermore, someone that wants their page to match the system font won't stop just there and they will try to match the rest of the page as much as possible to the target OS and then we end up with only one targetted OS because obviously there's no kind of fallback to say: If you're on El Capitan use these icons, if you're on Android use this set, for Windows users, use this one, etc...
Even the same set of icons won't fit different versions of the same OS, the same way that they state that it would be incorrect using San Francisco in the previous versions of Mac OS X because that's not the native OS font. Whenever any OS changes their designers update the rules about drop shadows, colors, gradients, rounding of elements.... some times in a radical way, sometimes adjusting and improving the experience, but everyone is able to tell if any app is following the current OS or it has been designed for a previous one.
In the same way, a web designed to look & feel like a native app will look horrible (kinda like Safari 1.0 did on Windows) in any other OS, taking away the portability and abstraction that the web brought us.
Using this for embedded apps is absolutely OK, there's no doubt about where such apps will run so it's just natural that they try to match the OS, but the moment that anyone uses this on web apps and the public web it's a kick to the groin of freedom, the ability to load any page with any browser in any OS.
To see what this is about, load the video from Apple with IE11 for best results: https://developer.apple.com/videos/wwdc/2015/?id=804