Problem with a non-android thing is that you lose the software platform it gives, as well as the ecosystem of apps. To do a new thing looks time consuming - the hardware and firmware has to be solid, the UI can't just be a desktop Linux in fancy dress, and then you need to write apps for stuff.

I sometimes think through doing a garmin-type thing with an e-ink display and the features I want - it works out to need way too much time: well-integrated hardware, decent low-power firmware, then the interface and custom map software is just a bit much for a spare-time thing for me (a pick one or two sort of deal). Android solves at least the ui and app problem in that case.