One of the changes by the FCC in March 2014 was that both the access point
and the client devices are each responsible for detecting DFS interference
from radar devices and, if detected, move off the channel.  
Prior to this, only access points were required to make that detection and
channel move, notifying their connected clients as to the channel change
so as to encourage the clients to follow.  This was part of the
original 802.11h amendment when UNII-2 and UNII-2e were opened up for
Wi-Fi.   The older rules made more sense from a Wi-Fi operations
perspective, as the client devices follow the lead of the access point as
to the channel that they are transmitting on.   Unfortunately,
manly client devices didn’t know how to interpret the
“I’m about to change from channel x to channel y”
message from the AP and didn’t move off “fast
enough”, which is probably what prompted the rule change.

 

The unintended consequence of this is that many consumer Wi-Fi device
manufacturers decided it wasn’t worth investing in the code to do
the DFS detection, and as a result just won’t operate at all on any
of the UNII-2 (52-64) or UNII-2e (100-144) channels.   This is
why many 802.11n consumer devices supported the UNII-2 and UNII-2e
channels, but their newer 802.11ac counterparts do not. Ironically, this
Is also true of EnGenius’s ESR wireless router series, and I
suspect the 802.11ac USB dongle as well.   Why this is true, I
don’t know, since EnGenius clearly has the DFS code from the
Electron and Neutron lines.

 

Fortunately, most phone and tablet manufacturers are not so shortsighted,
so iPhones / iPads and most mainstream brands for Android phones / tablets
with 802.11ac capability will work on the UNII-2 and UNII-2e bands. 
Also fortunately, most consumer client devices these days are dual-band,
so if they do roam to an AP with a 5 GHz channel they don’t
“recognize”, they will still connect on the 2.4 GHz radio
and be treated as a 2.4 GHz only client.  Where it becomes
problematic is using 5 GHz only consumer devices, such as USB dongles and
802.11ac wireless bridges (both of which I use in my house, which
necessitates me from not using UNII-2/2e channels on my home network).
   For myself, I’ll probably swap these items out
with EnStationACs in client-bridge or WDS mode once it becomes available.

Legitimate channels on 40MHz band:

The following eleven 40 MHz channels are legitimate to use for both indoor
and outdoor deployment with 802.11n APs and point-to-(multi)point:

 

  • 36 (36-40) [lower half of UNII-1 band]
  • 44 (44-48) [upper half of UNII-1 band]
  • 52 (52-56) [lower half of UNII-2 band]
  • 60 (60-64) [upper half of UNII-1 band]
  • 100 (100-104) [lower 1/6 of UNII-2e band]
  • 108 (108-112) [1/6 of UNII-2e band]
  • 116 (116-120) [1/6 of UNII-2e band]
  • 124 (124-128) [1/6 of UNII-2e band]
  • 132 (132-136) [1/6 of UNII-2e band]
  • 149 (149-153) [lower half of UNII-3 band]
  • 157 (157-161) [upper half of UNII-3 band]