Zigbee2MQTT supports more devices, and has better support for some devices that ZHA only has basic support. For example I have some vibration sensors which ZHA supports for basic vibration detection, but Zigbee2MQTT can set vibration sensitivity levels as well as detect vibration.

Zigbee2MQTT also has a more advanced interface with more configuration options, displays more information, and has better network visualisation.

Another advantage is that Zigbee2MQTT can be run on a different system from Home Assistant. That way the Zigbee coordinator can be placed in optimal location of the home, while the Home Assistant can be located elsewhere.

If you are already comfortable with using ZHA I there isn't much need to switch, unless you need support for devices only supported in Zigbee2MQTT or really need to relocate your Zigbee coordinator. Migrating a whole Zigbee network can be a bit of a pain. If you are curious about it you could always buy second Zigbee coordinator and run both.

I'm shocked nobody has mentioned it, but the primary advantage of Zigbee2MQTT is the ability to run Z2M on a machine different from HA. ZHA unfortunately doesn't allow for this, which is a shame because my Z2M box (RasPi) is centrally located but my HA box is not.

Some things that you might notice are devices going through battery much faster than you might expect, this has to do with the implementation of battery reporting on some devices under ZHA, I can't say who's fault it's it, but at least on Z2M you can even set the reporting intervals manually (if the device supports this)

Thanks for all the input. It seems that since everything is working fine for me I should just keep using ZHA. Maybe when I have plenty of free time I'll migrate to get the extra features but it's not something that I've missed.

Is there any difference in network performance, e.g. reliability of communication, or is that all down to the hardware?