Bishops Accountability, a U.S. pressure group, said the pope had apologized frequently for the harm done by priests but had never taken effective action to rectify the "incalculable harm" done to hundreds of thousands of children by predatory clergy.
"Benedict's words rang hollow. He spoke as a shocked bystander, as if he had just stumbled upon the abuse crisis," Anne Barrett Doyle, the group's co-director said in a statement.
The festering child abuse scandal broke out well before the then-Cardinal Joseph Ratzinger took office in 2005 but it overshadowed his papacy from the beginning, as more and more cases came to light in dioceses across the world.
Hundreds of victims came forward with devastating accounts of abuse suffered at the hands of priests sometimes over years that left them with deep psychological wounds.
The scandal broke in Boston in 2002 when reports emerged of the systematic cover-up of sexual abuse, with guilty priests being quietly transferred between dioceses instead of being stripped of their office and handed over to civil authorities.
The Irish church was also shaken by the revelations of years of abuse and denial in children's homes, which led to a diplomatic breakdown between Dublin and the Holy See.
Benedict spoke of clearing out the "filth" in the Church just before he took office in 2005 and subsequently expressed "deep remorse" for the damage.
But the shock felt throughout the Catholic world contributed to a steady hemorrhage of members.
"He publicly spoke about the crisis more than his predecessor but that alone is no achievement," SNAP, another abuse victims' advocacy group said in a statement.
"That's simply because disclosures of cover-up at the highest levels became widely documented during his tenure."
"DENIED REALITY"
His fiercest critics accused him of direct complicity in the cover up aimed at protecting the church's image from the increasingly vocal accusations of abuse.
"This resignation could not have come soon enough and it should be followed by the resignations of most of the hierarchy of the Roman Catholic Church," said Ray Mouton, a campaigner and author of "In God's House", a novel based on the abuse.
As head of the Congregation for the Doctrine of the Faith before his accession to the papacy as Benedict XVI, Cardinal Ratzinger had been responsible for investigating sexual abuse cases and formulating the Church's response to the gathering crisis since 1981.
He ascended to the papacy just after the high tide of abuse claims flooded into his office, which worked through some 3,000 cases of sex abuse between 2003 and 2004.
A strict conservative in other areas including in his attitude to homosexuals, sex outside marriage and the use of condoms, Benedict was accused of "an attitude towards issues of sexuality which denied reality" by the Austrian liberal church group "Wir sind Kirche" (We are the Church).
The scandal continued throughout his time in office. As recently as last month, the former archbishop of Los Angeles, Cardinal Roger Mahony, was stripped of all public and administrative duties after a thousands of pages of files detailing abuse were made public.