You make a very good point about buying quality gear!
Strangely enough, neither the Canada nor the U.S. have the choices of good-quality business-class FRS radios like you have in some of your high end PMR446 radios that you can get in the UK. Basically, all we have here are consumer-grade family bubble-pack radios that can be bought at the big box retailers. They are good for their intended purposes, but I would not recommend these consumer-grade radios for steady or business use.
I did not discuss the alternative of cheap Chinese-made Ham radios, reprogrammed to FRS frequencies because this is not legal, in spite of the fact that hundreds of thousands of people may have done this. Plus, so many users think they are like the walkie talkies they used to own as kids and they can simply turn them on and start transmitting on any old channel the factory may have randomly programmed in to them. They could be seriously interfering with legally licenced businesses or public safety frequencies. There are no Ham or GMRS radios that one can buy in the U.S. that don’t need a licence of some kind.
As for interference, I have never seen any interference with digital radios on any of our electronic or sound gear, but that could be because we don’t use too many digital radios on film sets quite yet. Rental radios are almost exclusively analog. (The same cannot be said of the cheaper Ham radios; I have seen analog Chinese Ham radios with RF leakage so bad that will turn on my dash-mounted GPS every time I transmit.)
The DTR and DLR Motorola radios that we can get in North America especially cannot interfere because they don’t stay on one frequency for more than a few thousandth of a second.
You make a good point about the importance of testing your gear and working with a good radio dealer who can swap radios if you find them interfering with any gear.