At the severe conflict end of the spectrum are rarely brought in to make a custody recommendation. Such cases are generally more obvious, with clear evidence of child abuse, mental illness or substance abuse.
What is more appropriate, and what we may be called upon to do is to address specific issues, such as how to protect the children, whether and how to go from supervised to unsupervised parenting time, to assess the treatment needs or the adequacy of treatment for one of the parties, and/or to monitor progress.
In such instances the mental health professional is more typically brought in as a parenting coordinator. These cases need long-term help; their problems may evolve over time, whether in a positive or negative direction, or roller-coaster style, with ups and downs. An evaluation is just a snap-shot.
Here is the conflict spectrum again: