I can't comment on OpenADK because I haven't used it.
I have used Yocto for three years and I would highly recommend it. I'm not an expert but they have very good documentation and that helps when you are starting.
Creating a new BSP is not difficult and they support Beaglebone
https://www.yoctoproject.org/downloads/bsps/morty22/beaglebone
I find their system intuitive and easy to add/remove packages, change the kernel or upgrade to the latest version. I also think that they are here to stay. In the last three years they have gained popularity and support. They are working on new tools that look quite interesting, i.e. Toaster https://wiki.yoctoproject.org/wiki/Toaster
I think it's worth considering.
Cheers,
Murray