SRR did have some difficulty against agile fighters as Randy Turpin who pretty much took his jab away in their first encounter in which SRR lost. He of course avenged that loss in spectacular fashion. An agile Carmen Basillo who was a natural WW would do a lot of bobbing and weaving to take away some of SRR devastating hooks in their first fight. In fairness, SRR did land enough jabs to cut Basillo's left eye.
His best defense was his legs, and perhaps in close phone booth fighting his defensive skill was not up their with some of the best Burley, Walcott, Locche etc. However, he was very durable. His only stoppage loss was due to heat exhaustion when he failed to get his 3rd divisional title against Joey Maxim. He pretty much collasped to his corner. He was clearly winning the bout.
Some will say his stamina cost him two losses to Paul Pender. But they were close decisions that many felt were hometown gift decisions for the Boston local. Moreover, SRR was like 39 years old at the time.
He really had no major flaws as a fighter. Perhaps he got hit more than necessary and could have finished stronger in some of his losses. But he is still #1 P4P in my list