With the limited play Buxton has received in the outfield, he hasn't had as much chance to shine in the field. In addition, with Buxton not anchoring the outfield defense, the two corners suffer as well. In the infield, if you believe Correa's rating, a lot centers on him. There is not a lot of speed on this club, particularly if Buxton is limited. That translates to a lack of range.
IMHO, the defense hasn't been that bad. They are mostly fundamentally sound--throwing to the proper base, taking the safe out when called for--but not catching balls that can't be reached will be a problem for this team.