been using for a while with no real issues, if you merge or move entities from one to another, those entities from the orginal device will be move to the other device and thus the original device can be empty ( imo this is expected behaviour), If you remove the device from the merge or remove the entities from the new devcie they do move back to original device with no problem.
in this example ii have merged a Zigbee wall socket and a Shelly PM mini so I get a switch and power readings in the one device.
the Shelly device now has no entities, but it is still required as it has the visit link to open the Shelly webpage, granted it would be nice for ALL info for all device to show in the Main new device, but this is not an issue really.