Be ready for the "Robby had impeccable defense but he chose not to use it because he was a real man who came to fight!!" apologists.
I wouldn't label his defense a "flaw". It was certainly helped by his foot speed, which allowed him to score plenty while not having to remain in the pocket for long. A sizable part of his defense was indeed his movement. I wouldn't call it anything less than good tho it was a level below his offense which was amazing of course.
One man who found a flaw in his game was Randy Turpin. When Robinson pulled back from one of his punches, he would throw a feint, and then the next one would connect. Perhaps the most outstanding case, of a lesser fighter beating a greater fighter, with a smart fight plan!
https://www.youtube.com/watch?v=GQ2d1Wpieak His defense was good, but he was more a boxer puncher than a defensive fighter, he really didn't have any real flaw, at his prime never lost, and fought the best.
Hell no, Robinson was an awesome in fighter, even as an amateur, he throws punches from every angle in really short spaces.
Had he been awesome infighter, he'd not have so much trouble against so many opponents (in the 1940s, in his prime).
He wasn't a "weak" infighter and i wouldn't call it a flaw as such. Comparative to the rest of his game it certainly wasn't a stength as such, but hardly weak.