I can't remember everything it broke, but I know some commands and reporting to/from the thermostat did not work, specifically with respect to fan operating state. If yours is working fine for you, I would advise heeding the old adage about fixing things that aren't broke. 
(On the other hand, I'm really curious if this problem was just me, but it happened after a reset and when paired to either Hubitat or ST, so 
.) Nothing super-important to me like setpoint-setting broke, locally or remotely, but I'm pretty sure I could only turn the fan on physically, and I ended up replacing the device since I had something available and am not aware of any way to downgrade.
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Thanks...yeah, my motto is if it ain't broke, it must belong to somebody else. 
I'm still deciding between the Zen and the T6. We are still bouncing back and for between severe heat waves, so I'm not touching anything on the HVAC until that is over for this year. The Zen looks like it would be a little easier for my wife to change set points on (slightly simpler up/down arrows) and it will look a little nicer on the wall (a little less chunky than the T6) both things that she will likely favor. Since my interactions with it will be 99% through HE, all I really care about is that it communicates status and reacts to commands promptly.
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"I assume they are zigbee and not wifi versions?"
Yes, Zigbee version. I've been checking the website and nothing since I posted unfortunately.
All good, Im happy with ~$50 each I paid for mine. I'm just waiting for Aus Post to deliver them, hopefully this decade! 
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I got my Zen Zigbee Thermostats yesterday and installed them - Im really pleased with them, they look amazing and work great. The wife thinks they look much nicer than our Gen2 Nests so that's another big win! 
The only thing I'm wondering is how I tell Hubitat that my Downstairs unit is Heat only?
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I think I found a solution:
That looked interesting, but I was wondering about what happens when you change drivers back to the Zen driver and don't hit Configure (I assume he didn't) and the Zen driver you end up on doesn't support those capabilities "left over" from the previous driver. Are those "left-over" capabilities then actually still available for use? Seems odd...
Hitting configure or refresh doesnt remove the "Supported modes" options.
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If that is an issue, couldn't you just set a crazy low cooling setpoint (as low as it can go, if there is a limit) and cooling would never enter into things. Right?
Well it's not really an issue anymore with my hack. 
So those additional non-supported capabilities are still available in RM etc, but they never did anything to start with so removing them from the Dashboard is the ideal solution imo.
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Btw, I just tried this hack on my Sensibo Sky (community driver) and it worked on that too! 
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Minor update: I was having occasional issues with my Zen's not changing attributes via automation and my units were both on v2.16 of the Zen FW. As there wasnt a way to update them via HE, I bought a 2nd hand SmartThings v2 Hub for $50 AUD and used that to update them.
The process is quite easy and you dont need to break all your automations and dashboards to do it.
- Take your Zen off the wall, wake up the screen if it isnt already on.
- Press the network button 10 times to remove it from your HE Zigbee network - this is only done on the device, your HE device won't be removed unless you run that process on the HE (so dont).
- On your SmartThings app, select add a device and pick the Zen Thermostat
- Press the network button 5 times to add it to your SmartThings
- follow the instruction HERE to check your Device FW version
- pull the batteries out of your Zen for a minute
- Wait for up to 10 mins and it should auto-update - you'll know it's doing this as a
symbol will start randomly flashing around the screen. It's done when the display goes back to showing "prefs". - make sure the Zen is responding in the ST app then power down your ST HUB
- Press the network button 10 times to remove it from SmartThings
- On your Hubitat Hub, go to discover Zigbee new device and start the discovery process
- Press the network button 5 times to re-add it to your Hubitat - you'll see it say something along the lines of "found an existing device". This means you succeeded! Congrats!
Hope folk find this helpful. 
How's did you figure out what FW version you were on?
If you take your Zen off the mount, select "prefs", scroll right to "info" and the first item when you scroll down is your current FW version.
Mine was on 2.16 and it's now on 2.32.
There are quite a lot of fixes it would seem:
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For the first time ever, I am wishing I had not gotten rid of my st hub...
Lol, I hear you - I never had one to start with. But v2 units are dirt cheap on eBay.
FWIW, it also broke reporting of the fan state (on/auto) for me. For a while I recall that it broke more, but some of that was just due to DTH fixes that ST needed to make (and did; this is when I was beta testing the firmware for ST) that didn't also affect Hubitat. But that's one feature that I think never came back for me on either ST or Hubitat. Not sure if I'm alone, but I ended up replacing the thermostat anyway so never ended up caring too much.
But for me this was one of those times I wished I listened to the old adage about "fixing" things that aren't broke...
It's working just fine for me on my C7, Im finding the Zen's much more responsive on v2.32 so far and haven't found any broken items.
I assumed that there weren't any major bugs left in the Zen FW as there hasn't been any FW updates since march 2019.
The other thing I have noticed is they are reporting improved RSSI numbers (as in lower).
PS, the only thing that doesnt work for me is the battery level - but that didnt work before the FW upgrade. I only noticed it wasnt actually 100% when I had them connected to the ST hub.
I sent a note to Zen to see if they had an outline of the update procedure. If I am lazy or bored and they respond, maybe I'll code something up. My Zen is on 2.08 
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Here's wishing you much lazy boredom.
