Nothing is wrong with the technology itself, just the way it is used.
I'd argue that they have been far more effective than any ground invasion, although civilian casualties are high as usual.
It depends on how you define effective, I'd say that American ground operations in Afghanistan have done far more to deter terrorism in the country than drones have. I will reiterate what most proponents of COIN say, that an insurgency is a political war that requires diplomatic engagement. Unfortunately, drone aren't very good at that.