It really depends of your device but some SOC have very poor mainline support and are essentially living in a forked repo with nothing sent to lkml, others have partial support in mainline but with some of the core features being broken.

Drivers can also be partially implemented / buggy depending on the device. (And I'm not talking about closed source stuff, even things like usb or touch screen drivers).

I would say that the major blocker is that nobody really cares about these devices and the task itself to mainline them is gigantic.

Most SOC have 3 or maybe 4 contributors maximum and the phonedev kernel mailing list gets a new patch only every 2 days. The whole phone mainlining community is probably less than 60 people.