There is a lot of discussion online about whether using Pifm is okay because it is low powered, or whether it is very bad because the square waves cause a lot of interference outside the FM band.
With something along the lines of a 30cm antenna, is that harmonic interference really noticeable? Similarly, regardless of how bad the interference is, have there been any cases of someone being contacted by the FCC or similar organization in response to complaints of interference?
Flying "under the radar" is nebulous at best. You are correct in that FCC is called in on interference. It normally does require a complaint (which I assure you that you don't want to deal with). However, with low power such as you intend, the chances of you interfering with an authorized broadcast are low. I was involved in a situation (years ago where a 100KW induction heater (10 kHz) had been reported as interfering in the AM broadcast band. The reduction in interference was approved by the FCC, problem solved. No penalties to the offending party. Perhaps go about your property, and those of the neighborhood and see if you can detect FM broadcast of your device. Being responsible party you have already indicated your interest. Make sure that no local emergency broadcasts by your neighbors (Home Alert necklace, etc are endangered). Go forth and sin no more
There is no doubt that operation is illegal. Regulations vary around the world, but in most countries operating unlicensed radio transmitters is illegal. In many places it is OK to operate low power equipment which is approved under a class licence. WiFi and radio microphones fall into these categories. CB radio and model control are also legal, even though using higher power, but only in approved bands.
There is no way Pifm would qualify, because of the out of band emissions.
Having said this, the chances of being caught and prosecuted is low.
Its 100% illegal, that's kind of like asking if its illegal to drive down the street without a seatbelt if you're only going 5 miles per hour. The answer is, if you get caught its illegal, if you don't get caught you won't get punished.
I've ran PiFM for a month straight without getting caught, but run at your own risk. If you're getting caught chances are you were interfering with some pretty important government stuff, as PiFM is a really messy broadcaster that spills over onto lots of frequencys.
So use good judgement.