[Big Dave]

Brayton cycle engines' thermal and mechanical efficiencies do not surpass those of internal combustion engines until you get to Frame 8 utility sized (400 MW(e) and up). Turboprops are indeed reliable (the Pratt PT6 is legendary - their MTBF is two pilots' careers) and very powerful for their weight, but are unmitigated fuel hogs at most altitudes. The LM2500 engines commonly used on warships are very reliable and easy to mantain but are nowhere near as efficient as big marine diesels.

In general, what drives engine design is available materials.