One of the things I've most been bothered by with the replacements is the "football knowledge" beyond just the rules. For instance, a team is trying to do a hurry-up offense and keep the defense from substituting. There is a potential issue that the refs need to deal with, do they understand the flow of the game well enough to understand that spending 8 minutes discussing a call is a problem or not. Thus far, I've seen examples of them not getting that.