Actually, during his first spell I thought his main problem as that if a ball was played in from the other wing, he would often be found ball watching instead of picking up his runner.
Wasn't that the season where we went a massive amount of games without keeping a clean sheet?
As you say, he does not make many clear errors like short back passes, miskicks or own goals, but seems to lack positional sense and awareness, and can turn off - not good for a defender.
Like Garmston was, he is good going forward and his long throws are useful but he is probably more suited to the role of wing back.