Wireless (Radio) interference even happens with WiFi-friendly devices.
Some wireless devices have been known to interfere with themselves. DWL-2700AP with both antenna screwed on and strapped to a re-enforced concrete wall will get less distance than that same AP with only one antenna screwed on (TX/RX antenna).
The reason the customers have different experiences of interference is because all the environments are different. Even in the same home. That is why you see manufactures listing so many things in their inference list. Some related to the band (2.4 GHz phones), some related to out of band interference (APC, Microwave ovens), and some related to physical signal interference (re-enforced concrete walls, sliding mirrored closet doors).
Please don't be discouraged that one solution doesn't fit all your customers. Trial and error is the only tool they usually have for placement and it isn't until they buy the product that they may learn they can not use it in the manner they wished.