You can't really criticize it only based on those numbers, at least not without looking at similar numbers from other cohort studies with the same length.
Well, one can't criticize the quality of the study without looking how well other studies did. But even if all studies involve a massive drop-out rate, the drop-out rate still seems like something to look at in evaluating the accuracy of the study.
I mean my concern, looking from the outside, isn't relative accuracy, but absolute accuracy. Not "is it as good as other studies?" but "it is good enough that I can rely on it?"