There have been numerous crash-bugs for the Windows font renderer, and even security exploits using it (especially before windows 10, as earlier than that font rendering was performed in the kernel's space rather than user-land). I wouldn't be surprised to learn of issues (at least of the falling over variety) in common Linux rendering engines and for other OSs too.
Yeah, go on IRC sometime and you’ll probably find out relatively quick.
Plenty of magic strings that break things on various platforms, hardly just an OS X thing.
A recent version of the OS + support lib added the possibility to reference a font in your app in order to have it downloaded (if necessary) and applied to your views.
IIRC It is restricted to one font delivery system only allowing fonts available on Google fonts. Not because of a power grab, but because fonts are not just graphics but also run a bunch of code.
So unrestricted access would have been a big security hole.
If you want more details, the font team talked about it at length during an Android Developer Backstage podcast episode.
Both of which are WebKit wrappers on iOS.