Today, Ubiquiti has pushed the latest version of UniFi OS for the UNVR and UNVR Pro which finally brings stacking support to the smaller device. It isn’t 100% clear if you can stack an UNVR and a UNVR Pro together, but that remains to be seen. However, what you can now do is stack two of the smaller UNVR devices together making it easier to manage more UniFi Protect cameras under one user interface.
This release also includes improved resiliency for UNVR flash storage, which was the case on older UNVR devices up until November 2020.
UniFi OS 3.1.9 for UNVR and UNVR Pro is available now, more information can be found here.