I've run mine in -10 F. It runs continuously for a couple hours until the trailer warms up in weather like that. I don't worry about "stressing" it.
They don't last forever -- there's cumulative deterioration of the burner and heat exchanger with every hour of operations, just like a home furnace -- but they do last a long time. Years, decades in many cases.
To learn to see below the surface, you must adjust your altitude