Unless you're in a very mild climate, then the gas pack is not as efficient. Putting the heating and cooling unit outside means you have a duct penetration through your perimeter wall. This must be well insulated and sealed, and even if that is accomplished, it will deteriorate over time. It is much better to put the air handler in conditioned space (even a garage is better than an attic or crawlspace). PLus the main plenum is outside -- cold in winter and hot in summer. Don't know how well they insulate them, but it is better if ambient temperature is closer to what humans like.
Evenness of heating/cooling depends on duct design. It would be very similar if the air handler was on the inside wall just inside of where the gas pack would be outside. Many houses are done this way (air handler at one end, long duct trunk along the house). I improved mine though by relocating the air handler to the center of the trunk instead of being at one end.