Hi Joe! At https://hostifi.net we have tons of people using the USGs and connecting them to the cloud server. The USG actually is the only UniFi device that has a web gui and it has configuration options for setting up the WAN before you adopt it specifically to solve that problem - how to get internet up so you can adopt to a cloud server.

If you have a couple static IPs at one site though, keep in mind that the USG can only do one static IP (without having to dive into CLI), so for any more advanced network like that most people are using a different router vendor but sticking with the UAPs and UniFi switches for the rest of the network.

We also have lots of people using both the built-in hotspot portal pages as well as external portals just fine.

As others have mentioned, if the controller goes down, the networks do not go down. All of the device settings are stored locally and don't depend on the controller, the only exception being that the hotspot pages do require the controller to be online in order to work.