When Statcast came out with infield defensive ratings at the end of 2019, Jorge Polanco appeared at the bottom of the SS list. Using actual game data, Statcast found Polanco was -16 OAA (outs above average). Compare that to Javier Baez of the Cubs at +19 OAA, and you have a 2+ win spread over a season of play. What makes their outcomes so different?
Russell Easom at batflipsandnerds.com takes a deeper dive into the reasons why top defensive SS are superior to those at the bottom. Turns out the key is making all the easy plays. Later in the article, Easom includes video and a discussion of Polanco's problems. He's a near-average defender on tough plays but can't throw a lick. A good read.
I think a lot of us agree that this just confirms the eye test. Polanco has the athleticism to play SS. But can he ever find a comfortable throwing slot?