If someone leaves a religion and then bad mouths it they are called an "Apostate". How reliable are their stories about the religion they are leaving? It's an important question because in order to accurately evaluate information you need to have a measure of the reliability of the source. Can you trust the source or is the source suspect?
When it comes to ex-members of religions, and especially new religions, religious scholars have come to very definite conclusions. You can read two papers by religious scholars here:
"In short, on the face of things, apostates from new religions do not meet the standards of personal objectivity, professional competence, and informed understanding required of expert witnesses."
The Reliability of Apostate Testimony about New Religious Movements (pdf)
The Reliability of Apostate Testimony about New Religious Movements - New Religion website
The Reliability of Apostate Testimony about New Religious Movements - Religious Freedom Watch
By Lonnie D. Kliever, Dr. Phil., Professor of Religious Studies, Southern Methodist University, Dallas, Texas, U.S.A.
"Neither the objective sociological researcher nor the court of law can readily regard the apostate as a creditable or reliable source of evidence. He must always be seen as one whose personal history predisposes him to bias with respect to both his previous religious commitment and affiliations, the suspicion must arise that he acts from a personal motivation to vindicate himself and to regain his self-esteem, by showing himself to have been first a victim but subsequently to have become a redeemed crusader. As various instances have indicated, he is likely to be suggestible and ready to enlarge or embellish his grievances to satisfy that species of journalist whose interest is more in sensational copy than in a objective statement of the truth."
Apostates and New Religious Movements (pdf)
Apostates and New Religious Movements - New Religion website
Apostates and New Religious Movements - Religious Freedom Watch
By Bryan Ronald Wilson, Ph.D., University of Oxford, England