Killing civilians is unfortunate and mistake. However it's controversial because the views of the leaders of a nation is quite different from average people that there are costs and risks you cannot avoid to protect more civilian death. Logically that's a legitimate claim. In statistics majority of killed were terrorists. i.e. the killing is quite effective and accurate.
The real flaw of the strategy is fundamental misunderstanding of root cause and how human society works. Killing didn’t reduce the capability of the terrorists. The growth of terrorists out paste target drone killing numbers. Killing cause more growth. That’s why US eventually talk to Taliban. The failure is on another level not accuracy of drone target killing.