In my experience, the only "safety" issues you can have with a gas furnace are with a cracked heat exchanger which allows carbon monoxide to be blown into the home. A gas pack is going to have the same problem. Gas furnaces are in tens of thousands of homes, and they go for years without any problems, as long as you have them cleaned and maintained regularly. Trust your builder; we don't know enough about the home you are building or which appliances you are discussing to give you an educated answer.
I found this which might help,
"Both a gas furnace and a gas pack use either natural gas or propane gas for heating and incorporate an electric air conditioner for cooling. Either system supplies warm and cool air to the house through a duct system. A furnace must have a split air conditioner added to provide cooling, whereas a gas pack includes both in a "package unit."
A package unit, either gas or heat pump, has all the components to provide heating and cooling in a single unit. Package units are located outside of your home, are weather proof, and are usually placed right up against the side of your house. Duct work is run from the unit (at the side of your house) under the floor in the usual manner.
A split heat pump or gas furnace with air conditioning is "split" into two sections. The "outside" unit is the condensing unit and is connected to the furnace or the air handler (blower cabinet) which is located under the roof (meaning out of the weather). The "inside" unit is typically located under the house, in the attic, or in a closet. Conditioned air is distributed through a duct system."