Apple has had a myriad of problems over the years with their WiFi implementations on many different devices. Their solution (plan?) was always to just point people to the AirPort or whichever Apple Access Point was the current model. This solution was less realistic with iPhones however because for the first time in history, Apple actually had a product with enormous market share instead of a small niche of dedicated users. So they had to be more diligent at getting the iPhone WiFi to work in a more universally standard way and for the most part they did but I still see rare instances where (esp a new version) iPhone has trouble. It will likely go away within a year or so when Apple finally decides to silently fix it in a software update. Because the problem really doesn't exist until they silently fix it.
And in all reality, WiFi has always been somewhat finicky regardless of the product or the network. Some are better than others. I'd love to know which chipset Canon used for their WiFi. That might explain a lot. It's possible that Apple changed the WiFi chipset in the iPhone 5 and it doesn't play 100% nice with the Canon chipset and/or that the iPhone 5 internal driver/firmware has a bug with that WiFi chipset. That might explain why the iPhone 4 worked fine. My wife's iPhone 4S and my Galaxy SIII both work fine with the 6D. My office and house WiFi and my Lenovo Intel WiFi all work with the 6D fine as well.