An electric heat pump needs heat available to pump. If it is 30 degrees outside, heating a spa to 102 or so, may be impossible unless the heater has some type of alternative heat source.
A properly sized, gas fired heater can take care of a larger range of heating needs.
A combination heat pump and natural gas heater, probably with a controller, really makes the issue more difficult, expensive, and would require a lot of study for efficient, cost effective operation.
Bottom line: In really mild climates a heat pump might do the job. In cooler climates the gas heater rules. And where a wide range of temperatures and high heat demand is wanted by the customer, a gas heater is the only way to go. A backup heat pump may take years to payout.
HM
Louisiana