I've probably ranted about this before, but marines in SC2 are probably the most overpowered unit in any RTS I have ever played.
50 mins each, can attack air and ground, they are produced quickly, they can be made 2 at a time, they have stimpack, and anything that counters them can only hit 4 at a time. They are all around superior in every aspect of their existence and anything that "counters them" are easily mowed down if there are more than 20 marines.
Anyone who even attempts to justify their fairness only does so because they play Tarren and don't want to admit they need to be nerfed.