In general, reliability drops as temperature increases. Several things happen, lubricants can break down and leak into places where they don't belong, plastics outgas and harden its a statistical thing. The decrease in reliability does not mean 100% or even 10% will fail, but it does mean that a failure is more likely. That's why elevated temperature testing is done with multiple units over thousands of hours, and periodically stopped to look a effects.
Much more important is the effect of rapid changes in temperature. Taking a product from 150 degrees Celsius to minus 50 degrees literally tries to tear a product apart due to thermal expansion and contraction. Parts can wear out mechanically from the stresses. Canon does test their cameras with thermal cycling. Its a excellent test to find the weak spots in a design in short order, a few cycles will usually cause a failure. Once all the weak spots in the design are fixed, the life time of the product will be greatly improved.
I was involved with a aircraft jet engine that was tested that way. Electronic boxes that are installed into a aircraft or space vehicle receive the same treatment.
The aircraft engine was run full thrust on the ground 24/7, stopping once a week for routine maintenance. The titanium in the engine turned blue from the heat, which was far above max operating temperature. Every time something broke, it was redesigned, or some sort of protection from the heat added. The best wiring insulation available cracked at far above its rated temperature, so heat deflecting shields were added. (the wire had max current going thru it) The concern for reliability was taken to extremes, and it paid off in super low failure rates.
