Hi Chris,
I've just had a look at a network with 60+ devices, a mix of Android (Google), Microsoft, Apple and non-standard devices and I noticed that static devices didn't have any connection failures but all the mobile ones (regardless of type) did. This is usually caused by the device roaming out of signal range before the connection has had time to complete. Do you see difference in behaviour from an iMac to an iPhone?
By centralised services, I was suggesting DHCP, DNS, RADIUS, VPN, etc. anything that is common to all your locations besides the Wi-Fi controller platform.
Have you tried a limited test to try clients on 2.4GHz and/or 5GHz only? Just to help rule out issues with DFS or other RF anomolies.
Thanks,
Darrel.