Not all, but a very large portion of the time I hear about complaints with 5GHz wireless, I hear "My iPad/iPhone/Mac does this..."
It's really rare that I see anybody mention (I'm not sure if it's lack of knowledge, or what) the issues Apple has had with wireless over the recent iterations of iOS/MacOS.
http://www.forbes.com/sites/gordonkelly/2015/04/09/apple-ios-8-3-wifried/
https://medium.com/@mariociabarra/wifried-ios-8-wifi-performance-issues-3029a164ce94
I'm not saying there isn't a Quantenna issue, or an improvement that could be made --however, I think that adequate testing at this point over 5GHz requires not using Apple hardware to do the testing. This at least eliminates one possible point of failure that people attribute to the RT-AC87 that might be the fault of other hardware.
My wife & I are Android users (Galaxy S5 & Moto X 2014), my kids are Apple freaks (iphone 5c & iPad 3rd Gen). While the Android devices have occasional issues (they're the only 5Ghz clients we have) the iOS products are practically unusable with the RT-AC87. The connection icon stays but there is no connectivity. Sometimes resetting WiFi on the iOS device will clear it up for a bit, sometimes not. So it appears that iOS is having issues for me even on 2.4GHz.
In short, my kids hate me for buying this router and are begging me to return it. I'm getting close to caving in.