I'm pretty happy with mine, put just a Unifi AP in at my in laws place with a cheap router. No USG or cloudkey, been working well for such a basic setup.
At my place I have 4 AP's, 3 different SSID's and different firewall rules between SSID's for communication between wireless networks. Different vlans assigned to different switch ports. Probably not complex stuff, but since I've set it all up I haven't had to touch anything or restart anything. aWhat doesn't work well though is IDS/IPS of my older USG, just not powerful enough. Also don't know if I can run an openvpn server on the unifi USG easily.
Anyhow, as for OPs request - the need for USB rules out a cheap'ish Unifi setup.