Spellskite can’t change a spell or ability’s modes (usually indicated by “choose one” or “choose two”).  Modes are locked in when a spell or ability is put on the stack.  If Steel Sabotage is cast choosing the mode to “counter target artifact spell,” Spellskite can’t change the target to itself, because the mode to “return target artifact to its owner’s hand” wasn’t chosen and Spellskite is not an artifact spell (while it’s on the battlefield).  The same is true for modal abilites, such as with the Exarch cycle.  Your opponent’s Tormentor Exarch‘s ability can give a creature he controls +2/+0, or a creature an opponent controls -0/-2.  Your Spellskite can’t redirect the +2/+0 mode, but can redirect the -0/-2 mode.  Make sure to read the chosen mode carefully to determine whether Spellskite would be a legal target for that particular mode.