Everything arm pretty much uses u-boot and the procedure is the same for each, but each needs it own targeted build of u-boot to work. Booting is the easy part. Generating the diskless image should be fairly straightforward, kernel may be a bit difficult to nail down if it is not available in a repository, then it's media. The cubox has a crapload of kernel patches that need to be applied to get the device to work. There is no current repository that holds them all in an already buildable (ie patches applied) state, so that's a fair amount of manual work.
Each of these devices will ultimately succeed or fail on their capbilbity to deliver media and control their connected devices. Accelerated video playback (including licensed codecs which could be iffy on a lot of arm) & HDMI-CEC are pretty much mandatory going forward. The cubox dual seems to have ok video support, a new lmce media playing device will be required. The banana pi seems to have some accelerated playback but I haven't really found much realiable information, regardless a new lmce media player is likely required.
With the rpi, and every other arm device at the moment, we lose the ability to play .dvd files, and real dvds, as we have nothing to deal with menu systems in the existing arm based players. Xine handles this nicely on intel architectures. I can sacrifice that as it is trivial to create playable versions, depending on laws in your local area.
The rpi is maxed out running basic MD software, orbiter & photoscreensaver use ~80%cpu with the pi @ 950Mhz. That's one whole core. qOrbiter is much better, it consumes ~40%cpu on the pi @ 950Mhz, and leaves room to launch other applications. Squeezelite takes 15-40% cpu on the rpi @950Mhz and cannot co-exist with a standard obiter & PSS. RS232 is horrible on arm as it doesn't have the same hardware access that intel hardware does. Serial control with GSD is a pig on resources and kills the rpi for nearly every other task.
Anyways, I'm not trying to discourage anyone here, completely the contrary. I am looking for 'THE DEVICE' to support going forward. We need something that is inexpensive and 'Just Works (tm)' when you plug it in. Almost anything that is dual or quad core in a 1ghz (or better) arm device and *has* good media support is likely a great device to investigate.