My experience has been that PE beams are one of the most reliable types of outdoor detection when installed correctly. Over the years, I have used nearly every type of outdoor detection technology (buried cable, microwave, dual-technology, fence detection, video motion, etc.) and still find myself coming back to PE beams when I need something I can rely upon.
I have never heard of insects being a problem, but wildlife such as deer and large birds will definitely trip a PE beam.
I typically use a stack of multiple PE beams mounted at multiple heights (8" off the ground, 18" off the ground, etc.) If it is a higher security application, I configure the beams in an "OR" configuration, where tripping any of the beams will cause an alarm. If preventing false alarms is of greater concern, I configure the beams in an "AND" configuration, where multiple beams must be tripped simultaneously in order to cause an alarm. The downside of the "AND" arrangement is that an intruder crawling down low would probably only trip the lower beam and would go undetected.
There are also configurations where the breaking of any single beam sends a "trouble" alarm, while the breaking of more than one beam simultaneously sends an "intrusion" alarm.
I am always leery of relying on unsupervised wireless links of any type, and suggest that you use a transmission system that is supervised, or notify your customer of the risks associated with an unsupervised system.