Now I don't have practical experience with a ground source heat pump, so with that disclaimer out of the way.... These systems should be less complex than a comparable air source system. The driving force with these systems is using the heat stored in the ground vs air so there is less variation to deal with. Right off the bat that means you don't need the energy robbing defrost cycle when the air temps are low. You are running an antifreeze solution through a closed circuit so you don't have to worry about the neighbor's cottonwood tree clogging up the outside heat exchanger.
The big variable with ground source is that the engineering and installation of the loop is critical as that is not pre-sized and tested by the manufacturer. However the tools and test equipment to evaluate the loop are common and inexpensive.
In a ducted system they are both equal in their complexity and unfortunately designers and installers have a pretty poor track record in executing this part of the system. The most efficient, magical box of engineering in the world can't overcome shoddy or poorly designed duct work. Again the tools for evaulating this part of the system "should" be standard kit with with a qualified contractor.
The accepted standard for sizing both air and ground source systems is called "Manual J". It works, but requires carefull and accurate input. Garbage in garbage will win in the end everytime. Down here, in Texas, systems are usually sized for AC load. An oversized system can be worse than undersized as failure to control humidity has led to more than a few houses being dozed due to mold infestation. Variable speed and dual stage heat pumps can be a good way to deal with the extremes.
In the end there are fairly simple and objective methods to measure the systems perfomance and there should be a set of inputs the contractor used to size the system. It's a simple matter of comparing A to B.
The subjective areas are the inputs to the "Manual J". A certain margin of error is endimic here as the HVAC Co. can't be responsible for the building envelope.
