In order for drones to be cost effective, they need to stay in the air for minimum the full trip out and back from the home, but if you look at the location of most big warehouses (e.g. Amazon, Walmart) they're built outside of big cities for cost reasons, so now you have to fly twice as far to deliver packages onto the opposite side of the city.

So either the FAA will need to allow liquid fuel drones (gas, diesel, etc) which have their own safety issues, or they'll need to be extremely large to hold enough batteries. Plus the recharge time of batteries will also be an issue.

I think weather and safety are their biggest detractors, but I think energy problems are next on that list. All these companies keep doing is pumping out press releases but without any specific details about how far their drones can travel, under what conditions, etc.

I'd short sell anyone who announced they're going to invest in making this work. It is fine as a pet project and to get some publicity, but if they start throwing millions behind it, I'm betting against them and it.