Pope Francis devotes a substantial portion of Evangelii Gaudium to homilies and their preparation (135-159). His final word is to be positive:
159. Another feature of a good homily is that it is positive. It is not so much concerned with pointing out what shouldn’t be done, but with suggesting what we can do better. In any case, if it does draw attention to something negative, it will also attempt to point to a positive and attractive value, lest it remain mired in complaints, laments, criticisms and reproaches. Positive preaching always offers hope, points to the future, does not leave us trapped in negativity. How good it is when priests, deacons and the laity gather periodically to discover resources which can make preaching more attractive!
Catholic Sensibility 