@Shwan Chalabi
Well, I agree with you. There are many factors which have influence on this game, but most of factors have tiny influence so it is really hard to find out if something has influence and how big that influence is. I'll try to explain it based on this example:
How to find out how useful Penalty Specialist is?
It's quite hard, isn't it? First of all, how many penalty kicks average team has in season? I strongly believe it's less than 10. And even if it is 10 or more, it still isn't enough to figure out how useful or helpful this Special Ability is - because player (penalty shooter) would have different performances in those matches and also there would be different keepers which also have different skills and performances. 1* defender can score from penalty and 9* striker can miss penalty so where are no certain borders.
Mine conclusion:
We shouldn't worry too much about single factors this game. Just play, relax and have fun. Don't forget that this game has primar task to earn money for someone, our fun is it's 2nd task