Its a real good question. I have the impression, that the special abilities are very effective. I.e. if you have a striker that is a Freekick specialist, he will do a lot of goals. The guy 2nd in my league has sucha player and he scored the most goals by far.
Shadowstriker is very nice for an AMC/AML/AMR
But I dont know either if I should train my Players and go for special abilities or just train them. Im going to doa experiment with my best two players who are 18 and 21, so I think everything below 22 is quite fit for such an adventure

Does the Heading ability also grant Attacking bonus, does it have a special combo then with freekick specialists?