I've played around with a few other boards besides Pi, and I'm always let down by the kernel support. A lot of them are stuck on ancient kernels or you're at the mercy of their BSP drops.

The Beagles at least have mainline kernel support, but things like pine64 and odroid still rely on a lot of patches or binary blobs.

Thanks for pointing out the X-15, though.. I've not looked at Beagles since the Beaglebone Black. It looks like an impressive board with mainline kernel support.