I think it goes beyond AC routers but the main problem manifests itself in the AC router at this time. At first the connection is OK even with the AC router but over time it degrades. Eventually the upload speed is so slow that the connection is eventually lost and "Limited" appears in the WiFi icon.
The only way to solve it is by doing a true shut down or to remove the WiFi driver. I even had this problem with the Linksys WRT 1900 AC and connecting to the 2.4 Ghz SSID was good at first but after a while (hours), it eventually got very slow or lost the connection. I was able to connect to my old Dlink router (non AC) and Surface Pro 3 had no problem for several days.
I am hoping it is not a hardware issue and this can be solved with software, but Microsoft support should be able to reproduce this rather quickly with any of the top AC routers on the market right now. I don't know how this one slipped by the quality control / testing department given the fact the the Surface Pro 2 and the 1 already had WiFi issues anyway...