All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If a Muslim mentions the faults of a group, it is not considered Gheebah [backbiting] as explained by the scholars. Al-Fataawa Al-Hindiyyah (a Hanafi book) reads: "If someone spoke against the people of a village in their absence, it is only Gheebah if specific names are mentioned."
However, it is always better to avoid this altogether and the Muslim should accustom his tongue to mentioning good things, leaving that which does not concern him. The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “Part of the beauty of a man’s Islam is that he leaves what does not concern him.” [At-Tirmithi, Ibn Maajah - Al-Albaani graded it Saheeh (sound)]
On the other hand, stating the mistakes of groups working in the field of Da‘wah is not blameworthy, but that should be done with the intention of offering advice to the Ummah (the Muslim community) and not for the sake of venting rancor. Please refer to Fatwa 82024 concerning the stance a Muslim should take regarding Da‘wah groups.
We make note that the violation of moral rights, like committing Gheebah, is sufficiently rectified through making Du‘aa’ for the victim, asking forgiveness from Allaah for him, and mentioning good things about him. This is because seeking his forgiveness directly could result in grieving him when he learns that he had been backbitten; this is, of course, the opposite of what Islam seeks of closeness and love between Muslims. For more benefit, please refer to Fataawa 81748 and 82148.
Allaah Knows best.