“Inept” is an unfair characterization of Montgomery. ‘Extremely methodical’ is probably closer to the mark.
‘Extremely methodical’ can be a very valuable commodity in a commander, as it was at El Alamein or in D-Day and the fighting in Normandy up until August 1944. On the other hand, in conditions of fluid fighting and rapid maneuver, ‘extremely methodical’ could be a great impediment, as it was in the pursuit of Rommel after El Alamein, the pursuit to Messina in Sicily, the Falaise encirclement, the pursuit to the Seine, and in Market-Garden as been pointed out. Maybe the most egregious example of this is Montgomery’s failure to clear the Scheldt estuary in September 1944 after capturing the port of Antwerp intact. The chances of ending the fighting in 1944 were slim, but they were zero without Antwerp functioning at full capacity.
Montgomery made two great contributions to beating the Germans: in 1942 - 1943 he showed that the Germans could be beaten in a stand-up and his organizational powers were absolutely critical to the immediate success of D-Day.
Of course, Montgomery gets a lot of bad press from historians because of his absolute inability to admit that any of his plans were less than stunningly successful and fulfilled to the letter.