For me is more of a con to have more white skills. It is harder to train certain attributes high, you need some grey skills for that. Because grey skills only grow at half the speed.
So if you have for example 1on1 finishing on a striker with tackling as grey skill and dribbling and finishing as whites, you can train finishing and dribbling higher because tackling is grey and doesn't grow that much. If you now have ST AMC MC, which I see some people use players like this, it will be harder to train some attributes high.

In my opinion there are only a few position combinations, that make sense and that I like. Those are DC DMC, ML AML and MR AMR and ST AMC also ok for the tactical versatility it adds. Other combinations I avoid. My advise is to never mix outside and inside positions and also not more than 2 positions.