There is no doubt that having a huge home market for Boeing and Douglas aircraft was an advantage. However, the European airliners of the time, the Bac 1-11, the Caravelle and the Trident all seemed to be a notch too small. The baggage compartments on the Comet and Caravelle were too small for one. The downsizing of the Trident and the reduction of engine size handicapped the BAC 1-11 and the Trident.
The DC-9 was a simple aircraft. Good hydraulic system design with redundancy. The electrical system generally was good but there was crowding of power relays and contactors behind the Captain. I suspect the Valujet fire in the bag hold cooked the electrical feeders that ran under the floor and therefore above the fire. Structurally it was robust with some problems with gear trunnions.
It did suffer from hidden icing on the top surfaces of the inboard wing skins. Chilled fuel in the tanks could freeze precipitation on the ground forming an almost invisible coating of ice.
The DC-9 being typically Douglas, used cables to move things. The only boosted control surface was the rudder which had manual reversion, The ailerons and elevators were flying tab operated. The elevators had a hydraulic down push in case of a deep stall situation.
The slats were operated by cables driven by a drum that was hydraulically operated. These cables had about the highest tension of any cable system I recall. The DC9-20 to 50 series had individual slat drives for each wing, not cross-connected while the little bit of -80 series I saw had one drive for both wings.
The engine installation was good and clean and quite accessible. The thrust reversers were the easiest to maintain of any I saw being the least complicated. They were good enough that Boeing fitted them to the B737's fitted with JT8D engines.
The folding cockpit jumpseat on the DC-9 was something else. I spent a lot of time on it as a Tech Service rep and being 6 ft 4 in tall it was a tight fit and you had to watch where you put your feet.
But I loved the aircraft. All the pilots seemed to like the Diesel 9.