The formalin bath should kill Uronema, Brooklynella and similar skin parasites.
I would not quickly discount it on the basis of one person reporting failure of the process. The dosage may have been wrong or the formalin wasn't stabilized and had become ineffective.
Another option could be that the parasites had already penetrated the slime coat of the fish and where protected by it. By its nature, a short bath of any kind can only reach the parasites that are on top or in the upper layer of the slime coat.
Fish highly prone to Uronema should probably be treated more aggressively with long term baths with either formalin or Chloroquine Phosphate.
From my own experience I know that formalin baths are effective against Brooklynella.
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