Just released for those CloudKey owners who haven't manually upgraded. Feature updates.

https://community.ubnt.com/t5/UniFi-Updates-Blog/UniFi-Cloud-Key-firmware-0-7-3-has-been-released/ba-p/2011908

5.5.20 has been OK for me so far. I tried the 5.6 series but it's still marked 'unstable' for a reason.