I don't know where this idea that the RPi has good hardware documentation comes from. One glaring example is its DWC USB controller. Sure, it has a Linux driver that is open source, but its datasheet is not publicly available!
So if you want to develop your own driver for it, you have to second guess its documentation by reading at the driver's comments. This is bad.
What do you mean by documented? Sure we have a general idea of how stuff works, and some implementations can even serve as a reference but almost nothing is documented in an official sense. Your average Chinese SBC is much, much better documented, in the sense that the SOCs are at least officially documented. The Broadcom soc isn't.
So if you want to develop your own driver for it, you have to second guess its documentation by reading at the driver's comments. This is bad.