Given the emphasis they placed on the defensive line in free agency, I tend to think the Bills believed most of their defensive problems originated from that group rather than the linebackers.
For the defense they play, I don't think it matters that much. They can be a middle half run defense, because their general approach is to jump out to a lead and force the opponent out of the running game anyway. Even the base defense they play...a 4-2-5...is geared toward stopping the opposing passing game. To that end, I think they're mostly satisfied with Edmunds being above average in coverage and ok at best against the run. He does what they need him to do and whether they retain him after this season is probably more about his cap impact than his level of play.