It is, but in my experience the reliability factor is pretty ugly for the rpis. I'd rather either pay directly for the device designed for it, or just run the software on a machine already on the network. Obviously that doesn't work for everyone.
RasPi reliability is tricky, for sure. The recipe that's working for me now is brand name power supplies (RasPi foundations, CanaKit, or Adafruit) and brand name SD cards. Don't trust SD cards that come in RasPi bundles or kits, even from Adafruit; I've had those fail horribly. So far all my Samsung cards are working great. I've also been using A1 grade SanDisks, no issues so far (only been a few months) as well as using cheap SSDs for one RasPi. I highly doubt the SSD is going to run into any issues, and it works seamlessly on the new Pis with a nice boost in performance.
The RasPi itself is so cheap I think a lot of people cheap out on the power supply and such, which ends up giving them lots of problems.
Anyway, given the poor reviews I've seen on the Cloud Key regarding hardware failures, especially on the newer versions, I just went with a Pi. But yeah, if someone has a machine on their network already, that's a great option too. I just wish Ubiquiti would release an official Docker image.
EDIT: I'm not necessarily arguing that people use RasPis for UniFi controllers, or that you personally should. I just thought your comment was a good opportunity to discuss RasPi reliability, because I know a lot of people have trouble with that.
In my experience, anything other than the Cloud Key is quite unreliable. I have been running it on my Win10 desktop without many issues besides the annoyingly restarting after updating.
