Yes there are lot of vulnerabilities for sure . No one can deny that . It is not yet perfected but it is getting deadlier.Drones can be countered with wide range frequency spamming. Like ddos attack on frequency range on which drone talks with base station. Drones can be hijacked to return to the home base etc. Drones are not the end all. It's just another tool.
Plus look at cost too. War is also economincs as much as technology.