I have no objection to ordinary speed cameras. If they are merely a form of tax (as is often alleged), they are the most easily avoided tax in the world. I have never paid it.
However, average-speed cameras are oppressive. They inspire fear in the drivers who are aware they are being watched and monitored, and that a mere error will be noted and punished. This Orwellian nervousness makes drivers more jumpy and irritable, leading to rash and angry behaviour.
Average-speed cameras contribute to congestion. A driver who has been accidentally driving faster than permitted then has to drive more slowly than permitted for some distance, holding up the traffic behind him. This again leads to angry and rash behaviour.
They also discourage drivers from concentrating on the road. The driver's primary focus is not the traffic around him, or safe navigation, but his speed. He is constantly having to take his eye off the road to observe his speedometer, since he is required to conform to a system that prevents his relying upon a mere mental estimate of his speed based on the traffic around him, and is having to concentrate upon what his speedometer says rather than what the other cars around him are doing.
For these reasons, average-speed cameras are probably counter-productive, generating less road safety rather than more. However, my primary objection is to the tyrannical insecurity they engender.