It's been a while since I looked at it. IIRC, the main problem I had was that many of the IO lines had shared/multiple functions and I couldn't find any documentation that explained how to switch them.

e.g. (contrived example), if a bit was MOSI for an SPI interface or DIO on port C, I had no idea which one it defaulted to and how to use it for the alternate function.

I got it to work for basic I/O, but it's obviously capable of much more.