I would argue the depth is better than thought, merely because it has had only very limited exposure so far. But in those very limited circumstances, the depth, especially Fleming and Karras have not embarrassed themselves or the team...
Fleming has had only a single start at both Left and Right tackle, and occasional snaps as the "sixth" lineman. And Karras has maybe two starts at LG. Games they played were victories.
This. Just like it takes an unusual amount of football intelligence to be a receiver in Brady's Patriots offense, IMHO that is true with Center also.
Who has had by far the longest tenure as Brady's Center? An undersized player (a college Rimington trophy winner) who could occasionally be pushed around by larger lineman but who was known for exceptional football smarts, Dan Koppen (I remember reading this somewhere but I'm too lazy to look it up now).
Now we have an undersized Center (a Rimmington trophy finalist) who can occasionally get pushed around by larger lineman, but despite his youth Brady calls him the "ringleader" of the offensive line, the guy who gets everyone in place. Sounds like a player with exceptional football smarts, perhaps guys like Koppen and Andrews work best with Brady.