For my employer we bought 3 Ubiquiti APs and this controller. With the installation of the controller I ran into problems immediately. We had the thing working quickly and adding the first APs and then we thought it might be inconvenient to have the thing in the basement. So unplugged and brought up. And then it didn't do it anymore. Reboot, reset, didn't help. Only a new firmware worked. After a lot of googling I found out that the thing is super sensitive to power outages.
Anyway, the thing ran for a week without any problems. A colleague wanted to see it work and noted that the APs are difficult to distinguish based on MAC address. After I named an AP the system crashed again. No matter what I did, after a reboot I got to the emergency page, but no reboot after that. The white light kept flickering. A brainwave was to replace the USB charger from a LG 5V 1.8A to an Anker Powercore 1 3.0A . After this, the Cloud Key came to life perfectly. The Cloud Key apparently sometimes asks for more than the 5V, 1A that is on the device (or vice versa, the charger does not deliver what it says). But it will cost you a lot of headaches and it would be better if Ubiquiti supplied a charger.
If it works then it works well. You have clear insight into the number of clients, which APs they are on, how much traffic they do, channels they use, etc. Guest page is easy to set up and manage. The APs themselves are very stable. If your controller goes down, only the guest page and so on will go down. The active clients continue to run.