I think the criticism is based on a confusion of what forgiveness is. The criticism also fails to distinguish kinds of anger. Forgiveness is not practiced in isolation from the other moral virtues, particularly justice. As a person forgives, he or she can and should stand up for what is right. Forgiveness and justice can stand side-by-side.
There is healthy anger and unhealthy anger. Resentment, an abiding, deep sense ill-will toward another, is an unhealthy kind of anger. In contrast, righteous anger, the kind that says, “You cannot treat me this way and I ask for a change,” can energize a person and help create justice. Forgiveness targets the unhealthy kind of anger, the kind that can destroy self and other.
When we make these distinctions (forgiveness in isolation vs forgiveness working with justice; healthy vs unhealthy anger), I hope you can see that forgiveness does not thwart justice. In fact, asking for justice without fuming anger might lead to a better justice.