With the advent of the digital age came a massive debate on privacy concerns/liabilities that came with such a massive connection of individuals that was the internet. Unmanned Aerial Systems bring this threat to privacy to a new level. To maneuver, drones use GPS and cameras, and realistically it would be impossible for the government or any private service to monitor all this data and make sure that it was disposed of properly when no longer needed to perform its commercial task. If private companies (say Amazon) begin to use drones, the amount of private information including video footage of private houses and the location of these private homes would be massive, and would immediately cause privacy concerns in the public that would take huge amounts of resources to monitor.
Say Amazon deliveries became totally controlled by drones. There would be 3.3 million drone deliveries every day around the world. This means 3.3 million chances for a drone to fail, and an accident to occur. Sure, accidents are just as likely to occur on the road for amazon drivers, but accidents including drones are much more risky. A falling projectile from the sky is more dangerous than the average car crash. And these safety concerns don't just apply to Amazon. Drones delivering medial supplies would be flying all over the place, carrying possibly toxic materials over commercial areas. Simple mistakes in programming or hardware could cause huge problems with drones.
This ties back into the first two problems of privacy and safety. Programs can be hacked by malicious organizations or people. When it comes to a chunk of metal in the sky that has the capabilities of obtaining information on buildings or people from the sky, it's safe to say that keeping these in the hands of responsible people is very important. It would be a huge safety concern if a hacker was able to control drones carrying medical supplies, for example, and preventing these supplies from reaching their destination.
If the world becomes too dependent on Unmanned Aerial Systems for transportation of important supplies, or for deliveries of purchases, it will be sorel disappointe when the important package the hospital needed or the package from amazon is delayed days or weeks due to inclement weather. Drones are greatly affected by wind, rain, snow, etc. Of course, a solution to this problem could be to keep other methods of tranporttion open in case of such weather, but if companies/the government become too invested in drones, they could be very disappointed with the results, due to the exclusive circumstances when drones fly well.