The Ubiquiti controller is not needed for general operation, unless you're using a guest hotspot. Otherwise if it's offline you just lose ability to do configuration and it's data/stats logging.
Hah, that's a dream world where enabling/disabling SSID's ever worked properly.
They have a good UI, good hardware but the software seems half baked.
Originally with the switch to the "new settings", the schedules were switched between the AP's and the UDM, not sure about a dedicated cloud controller.