The research report published by RAND says that regardless of U.S. or global policies concerning the export of UAVs of various sizes, these systems posing an incremental but growing threat to U.S. and allied military operations. Major potential adversaries—China, Russia, and Iran—recognize the utility of this capability and are producing many types of UAVs, including category I systems, for their armed forces (Lin and Singer, 2017a). Proliferation of these UAVs to potential adversary nations would, to some extent, complicate U.S. operations involving these nations. Given the continuing evolution and proliferation of both small and large UAVs, it is likely that, in future conflicts, U.S. forces will have to cope with adversaries equipped with different types and sizes of UAVs, both armed and unarmed.
The full version of the report can be read here: RAND Report on Drones
The report also available at the RAND`s website: https://www.rand.org/pubs/research_reports/RR2369.html