My experience in the semiconductor industry, and some knowledge of circuit boards says the following:

1) The higher the operating temperature for electronics, the shorter the life.

2) Temperature cycling (from cold to hot and back again, like what happens every time the car is driven and then shut off) is mechanically hard on circuit boards and the like. The constant expansion and contraction can lead to fatigue and failing circuit connections.

Ever notice that large mainframe computers are kept in a temperature-controlled room that is generally cool? It increases their reliability and life.

Putting a critical circuit board near a significant source of heat (and therefore, significant temperature swings) is a bad idea as far as expected reliabilty is concerned.