Cancelo's OK on the front foot, it's why he makes a lot of eye-catching tackles, he has the freedom to aggressively step out and meet people in midfield, but as soon as the ball is in behind him (which is quite often due to his positioning) he's atrocious and like you say, Pep avoids it at all costs.
I would take the Zinchenko-Dias-Stones-Walker backline oevey day of the week over any other. When Zinchenko is next to Dias his defending goes up a level, and he has the mentality for it, just lacks a bit of pace to be able to cover wide and tuck into midfield.
But when Cancelo is delivering goals and assists at the rate he has been, he has to play really.