I feel that the biggest drawback to a 20 mph limit is the added pollution that will be entering the homes alongside these roads. In Exmouth about 30 years ago, they built a new road to the docks and had to fit air filtration to all the houses on route to catch this extra exhaust pollution. I doubt the Government will pay out for that in Wales.
Of course, we will all be buying electric cars so 20 mph still causes no pollution. My own car would run at 20 mph on battery in Summer in these areas for a few miles then the 2 litre engine fires up to go uphill. At 20 mph, my engine is fairly high rpm with its cvt gearbox. Equivalent to second gear driving