I can think of two downsides: - Harder to get coverage. Both zigbee and zwave use mains powered devices (like light bulbs) as signal relays for other devices.

I can think of two downsides: - Harder to get coverage. Both zigbee and zwave use mains powered devices (like light bulbs) as signal relays for other devices. If you're using both, you need to ensure proper coverage for both networks. - No protocol native groups and scenes. At least zigbee allows you to set any number of bulbs to a certain scene using a single fast broadcast command. When using something like Home Assistant to unify both networks, recalling a scene will cause each individual bulb to be sent a separate command, which can be sloooow when you have many bulbs in one room.