That's the point, right? I'm sure there are much less people with mental health issues who are trying to legally purchase a gun than there are insane people intent on committing mass murder who can legitimately pass a mental health evaluation..
This argument says those that aren't violent but are labeled legally insane (because of things like anxiety) aren't allowed to buy a gun while those that are violent but are labeled legally insane are, and that's wrong fundamentally.
I disagree with it being fundamentally wrong. If I suspect someone of even maybe being insane, I don't want them holding a gun around me. That applies to anyone who fails a background check because of anxiety.
The only people I want to be legally allowed to hold a gun are people who know how to use it, when to use it, why to use it, and when NOT to use it, how to hold it, how to teach people how/when/why to use it, etc... People who understand all the safety features, people who understand trigger safety, etc.