I think it's a very basic rule of thumb that, if it takes longer for people to queue, figure out how the machine works, possibly have to download yet another app and manage to pay for their parking than the time they actually spend in the shops or facility for which they're parking, the council have categorically and unequivocally failed massively.
As you say, you just know that they will find any excuse possible to catch you out for a minor infringement and fine you - I'm sure they build these into their income projections and probably give the attendants targets. And if they do fine you, it won't just be having to pay double the parking charge, but guaranteed the fine will be far heftier than your infringement could ever reasonably merit.
I find it comical that the government are now making a big thing about clamping down on rogue private car park providers, when the councils are just as big rogues if not more! Just like when the government recently harshly condemned the petrol stations for allegedly profiteering, when the vast majority of the price you pay goes straight to them in tax, and then tax on tax!
Incidentally, now that the many councils that have stopped accepting cash for their parking machines need to pay far fewer staff to go around emptying them, have you noticed them passing that saving on to us by way of lower prices? No, neither have I...
In fact, the rip-off merchants will often charge you an extra fee for the 'privilege' of having to use their stupid app, when many people would never have chosen that method in the first place, rather than just putting in coins or scanning their debit card. It was enough of a scam when council car parking machines did still accept cash, but would never ever provide the machines that would give you change if you didn't have the right money. The most basic snack vending machines out there will give you change; but for 'some reason', this was 'not possible' for council car parking machines...