I can say about Garrett what I can say about any and all of these HC's, I don't know what they do when not on the sideline in a game and am not sure what some, including Garrett, do then.
I have a hard time seeing any NFL HC walking around the practice field and leap frogging his coaches and showing them how to do the job he hired them to do in front of his players. Most of these guys when they get to that role, except those pulling double duty, are more big picture, admin and the spokesperson for the team. Except the Cowboys have co-spokepersons.
I don't know how many of the positions Garrett can coach but his main job and the one I hold him at fault for is the in game sideline decision maker for the team. I do not doubt he sees what we see when it isn't working, I just don't know if he knows what to do and am not saying I do either and honestly, he has a lot of company in the NFL coaching fraternity or 20-25% of them wouldn't get canned every year.
What some of the more vocal Garrett critics fail to take into consideration is that in his time with the Cowboys, we could have gone through two other coaches with the same or worse results. The assumption that we would have done better with another or others is nothing more than that.