As above, BITD when bikes had horizontal rear dropouts to account for poor alignment, QRs could allow the wheel to slip if something went wrong. Almost all modern bikes have vertical dropouts and the force from the chain pushes the wheel's axle into a solid part of the frame.
There are a few cases I can think of where the rear wheel slipped during riding. It is much more likely on cheap bikes that have been converted to QR as the dropouot thickness is too little for the QR to squeeze on before it bottoms out against the end of the axle. If doing this conversion you must cut the NDS of the axle down by a few mms. It generally slows or stops the bike, but is not a catastrophic failure.
If the QR on the front wheel lets go, the results are generally much more severe.
So what bikes does this shop sell that have no QRs on their rear wheels? Do they disassemble the hub on every bike and install a nutted axle? Because every adult bike from almost every bike brand comes from the factory with F&R QRs. Only bikes with internally geared hubs, and single speeds, come with nutted axles.