Same here: APU2C4 running vanilla OpenBSD for routing, DHCP, DNS, attached to a Unifi AC configured in pure bridge mode. I was not too pleased with Ubiquiti's firmware though and flashed LEDE, for which you can find ready-to-use firmware images. No more need to install and run a java-based controller instance on another PC for just a wireless bridge.
For guest WiFi, I got a GL-AR150 for 20€ and also run LEDE, this time in isolation mode -- guests are NAT'd on their own network and cannot see each other.