It's a red flag law.
It can be abused to hell. If somebody accuses you of being a threat, your rights can be stripped by the legal system without a hearing. So suppose somebody has an ax to grind with you and lies about you being a threat to them... see where I'm going with this?
Nevermind the fact that it's a blatant violation of the fourth amendment's due process clause.
The NICS Denial system will spark a criminal investigation into you if you fail a NICS background check. About 90% of the NICS denials are false positives. You'll be automatically put on the defensive and have to spend money to defend your good name if the ATF/FBI gets it wrong.