I think his ego is often overstated, as with most popular views of famous people, they tend to focus on the most extreme of personality traits, often charicature get the individual - Eisenhower the film star, Patton the crazy horse, Churchill and his booze/cigars etc. Montgomery was likely dealing with Asperger's syndrome and when this level of self assuredness is combined with achievement, it is often seen as being a sign of arrogance and 'ego'. The thing is he was very critical of the performance of the US generals, which wjen combined with his turn shows him in a negative
I would also disagree with Montgomery being unsuccessful. Consider his position in the desert in 1942. The fall of France was characterised by Germaby having dedicatedd upto 25% of its economy tonre-armament and almost fully mobilising the nation in preparation for war with the western powers. It combined this huge military force with the operational gamble of placing it's strongest mobile forces into one sector. Due to French incompetence, and strategic serendipity the 'Blitzkrieg' of 1940 was successful. The west needed a reason to understand why it had lost so spectacularly, while Germany needed believed that they had found the miracle to rapid victory and the end of attritional war that was the guiding factor of industrialised conflict. What neither side really appreciated was that the Blitzkrieg concept was successful because of the situation of its use. Cue to North Africa, where both sides attempted to replicate this kind of fighting. While both sides would achieve operational success, this success always failed to achieve strategic success. Now queue Monty's arrival. He understands that the strategic success will only come when you can exploit the operational successes. The problem with armoured breakthroughs will always be the logistical tail and that is what stops an operational success becoming a strategic success. Thus Montgomery's strategy was not to punch through the German forces, but to engage him and defeat his force and not simply punch through, but to keep this defeated force engaged and keep advancing without allowing him to disengage, reform and regroup. This means slow advances that allow operational success to turn into strategic success. This is why the back and forth of the desert war finally ended in victory. No, it didnt have the deep armoured penetrations that capture the imagination of the Rommel fanboys, but it was hugely successful.