Well contrary to what everyone seems to think, they have been around for a long time - just not used in this niche market.
The ROI I think it not as good as say a box van. I understand there are only two companies using them, one has a great contract with two pharma companies and they have 7 of them running around.
There are limitations to them, mainly I understand with the "FedEx" system, they have limited sleeper room and limited cargo capacity.
Overall, with FedEx it seems to be a closed group for those fleet owners and special people.