mosfet amps CAN be less expensive to build, because the output devices can be driven from the voltage amplifier stage, and not needing driver transistors to boost the current. however, they often require several volts of gate bias, so the output doesn't swing rail to rail. bipolar transistor amps swing to within a few tenths of a volt of the rails. the crossover region is not as smooth a transition with mosfets as it is with bipolars, adding a small amount of distortion at low signal levels, those are the most commonly considered tradeoffs. there are two types of mosfets used in amplifiers, Vfets and lateral fets. the lateral ones require little or no thermal compensation, the Vfets do require a thermally coupled bias circuit the same way bipolar transistors do. laterals are much more expensive than Vfets (Vertical fets)
mosfets are more forgiving of difficult loads because they don't have a second breakdown region limiting their safe operating area. going into the second breakdown region with a bipolar is "instant death".