One red flag i noticed was when a company tried to guarantee results. The truth is no one can promise the IRS will cut down your debt by a certain amount. If you hear a guarantee i’d say thats a company to avoid right away.
My cousin used them and said they negotiated her payments down. She still paid a good amount in fees though so the savings weren’t as big as advertised.
My cousin used them for back taxes. They did manage to lower some penalties but it wasn’t a huge reduction. He still owed most of the balance. He said the process was super slow nd communication wasn’t great.