Before revision:  They differ  in the way they deal with lack of identification, in some cases solely through regularization (our proposed nuclear norm matrix completion estimator) and in other cases primarily through imposing hard restrictions (the unconfoundedness and synthetic control approaches).  proposed method outperforms unconfoundedness-based or synthetic control estimators  .
After revision:  They differ solely in the way they deal with lack of identification, in some cases solely through regularization (our proposed nuclear norm matrix completion estimator) and in other cases primarily through imposing hard restrictions (the unconfoundedness and synthetic control approaches). proposed method outperforms unconfoundedness-based or synthetic control estimators .