i think the categories nanopesticides and pyrethroids are difficult to compare. the first is about scale, the second about chemistry. Theoretically you could have a pyrethroid nanopesticide. also, 'safe' is relative to who or what you are thinking of - ie safe to humans via consumption? via exposure/drift? to water quality/aquatic organisms? wildlife/endangered species?

in my experience, it is often difficult to find a pesticide that is safe to everything and still effective enough to kill a pest. often mitigative practices are needed as well to prevent its offsite movement (such a vegetative buffers between fields and streams, buffer distances from water, etc)

I dont know that much about nanopesticides, except that the USA has only recently begun to take a regulatory notice of them. Much of nanotechnology (not just pesticides) has 'slipped under the radar' of regulation and permeated the market because it is not really a new material in most cases, just a new size particle. unfortunately, scale matters, and these particles can pass the blood brain barrier, etc. Thus, in my limited-knowledge opinion, i would suspect nanotechnology may become a big safety concern in the future. especially nanopesticides. But this is just my personal opinion.