The PT6 is a phenomenal mill, 60 years of refinement and perfection have squeezed out every last weakness from the base design. It’s reliable enough that the single tbp market has really cut into the twin market, because people feel dumb paying twice as much for about 10% extra capability.

The problem is that turboprops require high level upkeep and inspection to catch things before they go wrong. Even the best lycoming piston is just an air-cooled, easily parsed engine to the average mechanic. For turbines you have to send off fan blades for flux inspections and catch up on 60 years of mandatory fixes and repairs. The engine itself costs more than….most piston aircraft? You can get a cessna of pretty capable build, like a stationair or skylane for a third of a mil on the used market. Getting a new PT-6 after 2,500 to 4,000 hrs will cost about 3x that much. Overhauling it (or the Tbp equiv) will run you about….half a mil? Maybe? And it’s all over the map, because there are so many variations w/ diff. specs and requirements.

The engines are best for companies who can afford to run multiples, find some economy in scale, and learn THAT specific engine. Consistency lets them make the best of it, no more surprises or downtimes w/ a stock of uniform parts and prep.

This article is a pretty good rundown, usually the start whenever someone looks at the carrying capacity of a piston and starts asking about moving up to a caravan: https://www.aviationconsumer.com/aircraftreviews/aircraftstepups/pt-6-overhauls-complex-and-expensive/