Optimal temperatures are different for different components. - Unrecommended temperatures are always indicated in every technical document of the component (also, maintenance T, storage T, storage and maintenance humidity, allowed altitude). - Optimal temperatures - from the dew point (the lowest temperature at which condensation condenses) and up to 21-26C (usually, indicated in the passport). Capacitor is the most suffering element in hot temperatures. Electrolytic capacitors are widely used because of their low cost. You can take a look here what happens to them when they overheat. - Accordingly, unrecommended temperatures are the ones higher than indicated in the technical passport. - Using your devices in high (or low temperatures) may lead to premature failure (at high temperatures there is an accelerated degeneration of semiconductors and destruction of the elements) and change of embedded characteristics of the whole rig or its parts (the most usual example - decrease in the maximums frequencies).