It's not so much "harmonics of the FM band", but the wide band of spurs either side of the transmission.

The problem is that the unfiltered output of this device contains a wide range of spurs and harmonics which will radiate right across the VHF and UHF bands.

Plus the antenna (a few feet of wire) will have multiple resonances across the same range.

It will radiate hundreds of signals, and the odds of them interfering with a nearby essential service will be very high. And note that the essential service receiver is capable of hearing signals in the micro-watt range and is routinely trying to receive them.