I doubt it's worth it to buy Android devices for crunching (phones, tablets etc). As I mentioned before, battery problems can happen easily. I'm only thinking about doing it with my old Moto G because I feel it would be a waste to have a nice quad core chip laying around doing nothing when it could so easily be crunching (and it's the best, actually only device I can do it with)... But I am wary of the battery. I recall raising the maximum battery temp with my ZTE Force... Won't be doing that again, may even lower it.
I might just end up throwing my old ZTE Force out, it has too many problems. The place I worked at when I had it ruined it (the speaker), and now the battery is messed up. Unless I can run it without the battery...