HI Jas,
Private revelation isn’t necessary, because it isn’t telling us what public revelation hasn’t already told us.
Joseph Smith and Muhammad invented a new faith tradition from their mystical experiences (and personally, I believe their experiences really were supernatural in origin, but I don’t know that as an empirical fact) but mystics in the Church aren’t inventing a new faith tradition. They confirm what has already been taught for generations.
C.S. Lewis presented a case towards Jesus, that he was either a liar, a lunatic, or telling the truth. Since he was accepting being actually worshiped by people and calling himself the salvation of the human race, he was either a crazy person, or he was a nefarious liar, or he was telling the truth. But it’s not possible to just call him a good teacher, because a good teacher would never be worshiped as God unless he actually were God. It would be scandalous.
IMO I think the same applies to canonized mystics, because unless they really did accurately report their mystical experiences, the only alternatives are that they were delusional people with good intentions, or wicked liars. I don’t know how you can make a case that they are good, sane people, who said the things that they did.