Might be a chipset compatibility issue, since all of those devices use Broadband's wi-fi chip architecture for 802.11ac. Issues like this are very common any time you introduce a new wi-fi standard, and all of the routers and client devices have to maintain backwards compatibility with an increasing backlog of devices.
Even (especially) with a newer router, the device manufacturer might have already posted several firmware updates before it even makes its way into your home. Check the firmware version on your router, you might be surprised at how many versions have come out since you bought it. In the years I had a Netgear router, it had several firmware updates, yet still had problems with some of my devices. It wasn't until I kept everything on 802.11g only that things ran somewhat reliably.
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I haven't seen those reports at nearly the same rate as the 6, 6 Plus, and iPad Air. The 5, 5c, and 5s also use Broadcom-derived wi-fi chips, but those are older designs that do not support 802.11ac.
Like I said, FWIW I just did a wi-fi test today and came up with nearly identical results as one I ran last year with iOS 7.0.1. Same phone, same router, same internet service, same location.
