My primary criticism of Grandal was on Hahn, signing for four years (and with no other reported offers) was too long for a catcher in his 30s. Few catchers (5-10%) are able to play regularly once they cross 30, and their production typically suffers a significant drop off. It's hard to see him catch more than 70 games his final two seasons here, which would be slightly below his pace the first two seasons, though that is to be expected at his age. However, it doesn't appear to have been considered in his contract offer.
The one area where I think he could have improved on, and perhaps he did, was to take on the game / pitcher prep, having McCann as a mentor in that regard.