I had to revert as all the latest firmwares cause Wifi connections to either keep dropping or in some cases devices can't connect. Downgraded to 4.4.11 and it's fine.

Anyone else experienced this?

Yes, same here: in some cases devices can't connect or loose their wifi connection. Especially true for devices connected after the upgrade.

I'm running a 2865ac on 4.5.1_BT and don't have any issues with connections dropping.  However, I have noticed recently that if certain changes are made to the wireless configuration then devices which previously connected without a problem will no longer connect until the router is rebooted.  I don't recall having to reboot this way on earlier firmwares.