Are piezoelectric energy harvesters too expensive to be used commercially as a power source in drones or small aircraft?
What are the disadvantages of such a system being used as an auxiliary power source if not the main one? Is it not worth the money?


It's not worth the weight.

The power that can be extracted from piezoelectronics is on the order of microwatts (100 uW in the above article). In contrast, light aircraft have their power measured in kilowatts and commercial aircraft reach a megawatt of electrical power.

In other words, it would take 8-16 million of piezoelectronic harvesters to replace the alternator on a small aircraft, and 3-10 billion on a modern airliner. Drones use more power than small aircraft as they've got a lot of electronics.

Piezo harvesting is being developed to power completely unattended tiny sensors that won't have access to any other power source for years. All aircraft, including drones, already have their own power source - the engine - with fuel tanks that get refilled each flight. So power's cheap and available.