The longevity and reliability of a heat pump is independent of the backup heat. The heat pump will operate down to an outside temperature between 35-40 degrees regardless of the supplimental heat. So the amount of time the heat pump runs depends on your weather, in your area, and the transfer temperature set point.
Furnaces that use duel fuel heat do tend to have higher performance values than most. They cost a little more money but it is definitionaly worth it in the long run.
Yes, the fuel heat pump furnaces which are based on duel fuel or duel power are more reliable than the ordinary ones running on single fuel or single power. The former ones have a longer life span so it is worth spending extra cost.