They are like hit and miss; some seem work on some players and the same do not on other players and some others (for example, dribbler) are completely unoticeable (probably useless). In another way to say for the hit and miss thing, you might buy a player with free-kick special ability or corner specialist (or train a player yourself the special ability) and this player will perform well at free-kicks or at taking corners but the player might also be a flop in free-kicks or in taking corners. You can buy a defender with free-kick special ability and he performs well at free-kicks but you can also get a midfielder or attacker with the same special ability turning out to be a flop when it comes to taking free-kicks. There is no guarantee of the so-called advantange. It might also be more like a placebo thing rather than having any real effect other than visual.