I think it's mostly that mid lane is snowbally and there are a mix of people who are true mid mains (and have played their mid champs for hundreds of matches) and people who are trying to get better at mid (and have played maybe 20 matches with that champion). As soon as the better player gets a bit of a lead (or a jungler camps the lane for a lead) the mid laner can typically impose more impact on the rest of the map than any other role.
But yea, whining that a champ is OP seems pointless. Some are strong, but at this point in development Riot knows what they're doing champion-wise and they're relatively balanced, with win rates fluctuating plus or minus 5% based on how the current playstyle favors some champion's kits over others (and itemization, etc...).