I haven't tried any of their other products, but the iMX6 has a very large detailed technical manual (publicly available, not under NDA) and their Linux kernel trees are updated in public via git (presumably not everything is public but it's a lot more transparent than the occasional tarball source drop approach), plus they have people involved in yocto development. Better than many/most of the ARM SoC manufacturers.

The only undocumented/secret squirrel business AFAIK is the Vivante GPU. And that's universal across ARM Soc GPUs, sadly.