The hassle with anything attached to the rear is how it interacts with the PDC sensors, and potentially blocks the rear-view camera. While it is not enforced much in the USA, in other markets (say the UK), fitting a bike carrier on the back that might block either the lights or license plate would require an auxiliary light/number plate hanging off the back to provide unobstructed lights and proof of registration. That would also require fitting a wiring harness for their use. That could get very expensive. I did that on my ICE, but on my i3, I carry mine in the rear. Safer, and not open for damage or theft in the process.
2011 535i x-drive GT, 2014 i3 BEV