The downside to a red flag law (especially for mental health issues) is it could discourage people with mental health issues that own or desire to own firearms to seek help.
Some states already require this, but it isn't a perfect approach. At the same time, when the Supreme Court is essentially whittling away the federal right to privacy, one can't reasonably go and argue out the other side of your mouth that your social media accounts, mental health, etc., is subject to a right of privacy that may not exist federally.
A few of the states red flags can be denied by a sheriff, but there is legal recourse, but it can be timely and expensive, and the process varies if it was denied at the state level (mental health or local domestic violence, etc.) or at the NICS/FBI level.
