They are not design-only, all these factors do matter. The foot, the age, time in the team, and even which players play nearby in the formation - that affects performance of the given player. There's much more dependencies and nuances in this game than anybody knows about, including even the developers. Nordeus keeps very good secrecy where each developer knows only small part of the whole picture, and very few, if any, see it in full.
The problem is their decision to make it all a mystery (an important keyword in their philosophy) to the users, and allow each user to discover everything by experience. They think, doing otherwise would make the game too boring and calculated, plus the fact that you can't calculate much in real life either. But when your fully prepared team with top morale and health loses 0:3 to the team 10 points weaker, whose manager did not show up, which got red card and two injuries that were not replaced, Nordeus expects you to be excited by the wonderful unpredictability of it, so relax and enjoy - exactly like several people have suggested in this thread.