We did extensive testing in the climate chamber and felt that going down to -40°C is safe.
The BCM2837 normally runs at 1200 MHz and the firmware starts downclocking when the core temperature exceeds 80°C. Once it reaches 85°C, the frequency is reduced to 600 MHz. It will be further reduced to 300 MHz when going beyond that. In our testing we found that the CPU is eventually halted when it becomes too hot, but can be rebooted without any issues once it has cooled down.
If you know that ambient temperature regularly exceeds 55°C, it may be necessary to install cooling in the cabinet together with the RevPi.
Since you're mentioning street cabinets, our customer Nicolai Buchwitz of Enda KG regularly installs our products in street cabinets for monitoring of hydrogen fuelling stations:
https://twitter.com/NicolaiBuchwitz/status/11127278200427274...
Does the hardware watchdog work when halted because of over-temp?
One particular product in the Revolution Pi lineup called "Connect" has an additional hardware watchdog which is also capable of resetting attached devices via a relay. That one works even when the BCM2835 has locked up completely. However, the CPU needs to have cooled down a bit to reboot successfully.