So, in my results, I found a few interesting points:
(Please note that these are my observations, and your mileage may vary!)
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I found that almost all zigbee contact sensors that report temperature, "freeze up" in a chest freezer where the average temperature is between minus 5 and minus 10 (Centigrade).
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No issues whatsoever, for some sensors in a standard fridge (Visonic MCT-370, Ecolink 4655BC0-R).
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For all other freezers (e.g. stand up models), covering the sensor with scotch tape increases its lifespan by an incredible amount. I still have an Ecolink going strong 6 months after placing it in a busy fridge. (The scotch tape covers the sides of the sensor, not allowing the cold air to get inside into the inside of the sensor).
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I have found that the (relatively new) Sonoff USB Dongle Plus (P version) is an incredible repeater. Positioning it as close as possible to the sensors has made a huge difference to dropout. My theory is that the transmission from these sensors is relatively weak, so this has to be placed close to the sensors, not the Hub. (Others may disagree as to positioning of these devices. In general, 5 sensors = 1 repeater).
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I'm not sure if it made a difference, but for all sensors, after initially pairing them, I used the "zigbee reporting configuration driver" from @kkossev (Thank you!) to set the reporting interval. Your mileage may vary, but at least its an attempt to get the sensors to report back regularly.
View and Change Zigbee Device Settings - #7 by arnb -
To small commercial owners, reporting "open" AND reporting "temperature" is key to their success, so the generic zigbee contact sensor is the preferred type of device.
