The primary drawback to the 5th wheel type of trailer is the loss of the bed of the truck for all practical purposes whether or not the trailer is attached. With a conventional trailer the truck can still be used to haul a boat/kayaks/canoes or bicycles or extra gear or even an ATV.
The 5th wheel arrangement is required for very heavy trailers that exceed the capacity of a conventional hitch. The 5th wheel trailers provide the elevated sleeping area and as a result have a higher profile and more wind drag going down the highway and most involve a higher base floor level as well so more steps involved getting into the trailer and again to the sleeping area. They also often provide more externally accessible storage with the basement area than a conventional trailer of the same length.