Most ignore qualitative criteria such as weather, participation (or lack due to injuries or "throw-away" games) and individual efforts.
Goals of some rating systems differ from one another.
Rating systems cannot ignore "throw-away" games. These are games where teams have already earned a post-season bid and have secured their playoff seeding before the end of the regular season, and want to rest/protect their starters by benching them for those remaining regular season games. This usually results in unpredictable outcomes, but without a mechanism to ignore such games (which runs counter to the goals of such systems), this will unintentionally skew the outcomes of rating systems.