So far, the integration is working great for certain devices, but one hole I've found is that if I have the thermostat mode set to "Auto", it doesn't show the heating & cooling sliders separately and it's impossible to set the setpoint of the thermostat. If I set it to Heat or Cool, I can then set the setpoint.
It's definitely possible to have this work as it works OK from HomeBridge, so thinking it might just be a miss. I tried both the generic Z-Wave thermostat driver as well as the Honeywell specific ones with no change in behavior.
I have a T6 as well, and I switched it to the "Generic Z-Wave Plus Thermostat" driver, and now it works in homekit. (I'm having trouble with scheduling, but that's a different issue, probably).
Edit: Now it works fine with the "Advanced Honeywell T6 Pro Thermostat" driver too. So, maybe it was moving to .116 that fixed it.