A clever idea that allows your network to be separated from your server installation.
I am currently testing this out at home with a USG, UniFi Switch and single UniFi AP.
But I am likely to deploy at work in the near future, as the cloud access, ease of configurability and removal of the dependency on a server to run the UniFi Controller software is very attractive.
Configuration of a valid SSL certificate for the cloud key is a royal pain in the console that I haven't attempted as yet. I will wait for the GUI to be implemented before I purchase devices for work. Also, the number of usernames and passwords that you are asked to create on setup is quite confusing to the first time installer. Changing from the default IP address range is also a little trickier than it needs to be - I guess this is because the UnFi interface is attempting to dumb down network configuration tasks, where most home users would be happy with 192.168.1.x as their subnet without being asked. That's not the case for business VPN users unfortunately.