Raspberry Pi is a much more proven platform for serious hardware developers

Are you saying proven because it is built better or proven because it's been around for longer and has more documentation ...etc?

I find UDOO (ideally) as a great platform to start, given that it seems to come with a lot more connections to hardware. However, the lack of any documentation makes it a bit hard to start.

Although, they do have a skematic for the tech, so technically I should be able to figure it out ... if only I had a spare day or two to spend on it :(

I would say mostly the latter (been around longer, more docs, etc). Power consumption on RP kind of sucks though