For very low temperature monitoring, down to -40 C, I would consider the kuomostat wireless sensor tags. These are neither zwave nor zigbee, but seem to do really well as single value sensors across a wide range of temperatures. You do have to also buy their receiver unit. There’s no direct integration with SmartThings, but a number of community members have created indirect integrations. They do have an IFTTT channel, for example. And an open API. Just nice little devices. (Although I understand customer service is pretty much nonexistent.)
I’ve seen the Wireless Tag sensors. But this is basically the same problem as my NetAtmo setup, It’s a third party system and as soon as they will change something, the integration will break.
I’ve had too much trouble with these third party integration (NetAtmo, Logitech Harmony, Belkin Wemo…). I’ve decided to try to stick as much as possible with Z-Wave or ZigBee only device in the hope of avoiding those problems.
I understand the concern, the problem is that most temperature sensor devices using the home automation profile are battery operated, and the typical batteries just won’t operate at that level. An IFTTT channel integration is less likely to break unexpectedly than an API one.
Several people have reported being able to successfully use the ST multisensor inside a freezer. Not as extreme as -40, but have you looked into those?
I used an AcuRite sensor outside for years and now a NetAtmo, both are battery operated (2x AA) and seems to have no problem reporting temperature below -35C.
If I look at my Aeon Lab multi sensor, it stops reporting temperature below -5C. The other sensor (luminescence, motion) continue to report, but the temperature just stops when it gets around -5C. When contacted, they said that their product wasn’t designed to support that kind of temperature. The spec sheets state that clearly but I was still hoping.
Every Z-Wave device I’ve found so far seems to stop around -10C. All the sensor that can go below this needs some kind of third party integration (eg: NetAtmo) and these aren’t reliable enough.