It’s all Ikea stuff.
I recently dealt with this on a fresh install of Home Assistant, ZHA, and a Nortek stick on latest firmware.
In my case, I was seeing NETWORK_BUSY notifications with about 50 total devices, mostly Hue Bulbs of varying form factors.
I ended up tracing this down to two issues:
- the
Adaptive Lightingintegration - I was targeting ZHA groups for lights, which apparently don’t support some of the features Adaptive Light looks to use. Targeting individual bulbs seems to be the fix here (see open Github issue: Controlling one bulb in a ZHA light group affects all the bulbs in the group · Issue #345 · basnijholt/adaptive-lighting · GitHub) - Automations - I had two types of automations I was using to turn on and off ZHA groups using various Pico remotes around my home. I had one of these automations I was developing spamming the ZHA groups causing NETWORK_BUSY.
So in the end, end user error, although I did consider moving to Z2MQTT initially.
What did you use for a usb stick with Zigbee2MQTT?
I switched this past weekend from using my Hubitat to using a SkyConnect with ZHA and it … hasn’t been great. I’m going to get a longer extension cable to see if that helps but if not, I guess I (and my 75 Zigbee devices 
) will have to think about swapping. Again.