UBRs take LiPos in the tube fine - proper magpul ones do anyway.
I've fitted a 1300 11.1v and ascu eletrics in with the bottom of the battery sticking out slightly. Currently run a 1450 7.4V with standard wiring dead easy.
The UBR will be more unique and are generally more sturdy in extended positions, but can be a little bit of a faff to change batteries/extend/retract the lower section.