Weight of Guardian Ad Litem Report

I am working on a brief dealing with the changing of a guardian ad litem report in the middle of trial.  This is rare and I think prejudicial.  However, after Gateley v. Gateley located here, the biggest question is what defects, if any, will justify reversal.   This is what makes me wonder if it is not time for the chancery court to consider court-appointed experts in complex custody cases to minimize potential problems and defects in GAL cases.