The strongest argument of the armchair critics is the disposition of the British 1st Airborne Division at Arnhem. Only one battalion was able to get into the town and secure the north end of the bridge. The rest of the division landed about 3 miles west of the bridge, and were cut off from reaching it by German SS troops. The critics argue that the plan for the 1st AB was too cautious, and contrast it with the aggressive use of German Falshirmjagers in 1940, particularly in the capture of the Belgian fortress of Eban Emael, where gliders landed directly on top of the fort.
The second argument is that British 30 Corps was also too cautious, and therefore too slow, in its drive north to relieve the airborne divisions holding the bridges. It's generally accepted that the Brits were more casualty averse than the Americans, as they were experiencing a serious manpower shortage by this time in the war. The argument is that the Americans under someone like Patton, or the Germans under just about any general, would have been more aggressive and might have got to the south end of the Arnhem bridge a day or so sooner. But that is speculative, as German counter-attacks against the narrow corridor caused extensive delays as 30 Corps repeatedly had to concentrate in order to clear the single road north.
In the end, it was an overly ambitious plan, executed too cautiously and without adequate reconnaissance. None of this diminishes the heroism of the British, American and Polish troops who fought the battle. In fact the bridge at Arnhem, later bombed by the American Air Force, has been rebuilt and is named the 'John Frost' bridge, in honor of Major Frost, who commanded the battalion that held it for four days against overwhelming odds.
BTW, the scenario has an option in which the entire 1st AB lands adjacent to Arnhem, instead of 3 miles west. It is considerably more likely to produce a successful outcome.
Ps. If you're interested, you might want to watch the 1977 Richard Attenborough movie, "A Bridge Too Far". It's three hours long, stars just about everyone at the time, and is one of the best war movies ever made.