The depth is the biggest problem, these guys on the bench aren't exactly momentum keepers when they're in the lineup or PH for someone.
I dont remember which analyst said it, but he mentioned that a good bench player is someone a manager can count on to just help keep good momentum going for a lineup when they get an opportunity to start, whether it in the field or at the plate.
Cubs outside of Catchers, they don't have that guy they can count on consistently on that bench, you're just hoping they don't lose the game in the field or kill rallies when they bat.