I wouldn't call it 'extremely poor'. 'Dubious', maybe, or 'questionable'. Or simply 'bad'.
Furthermore, as far as I know the exercices used in the 'strengthen' units are taken straight from the regular units. Duo should keep score on those words and sentences you make the most errors on, and present these in the 'strengthen' exercices.
On the other hand, Duo certainly isn't perfect, so it's perfectly possible that what you described happens now and again.