Worf: Had really bad luck with women.
O'Brien: Had really bad luck with everything but women.
Harry: Had really bad luck with everything!
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Be that as it may, I think Harry was quite underrated as a character.
1. He was a good friend, and fiercely loyal. In "Non Sequitur", what would have stopped him from just saying "screw it"? Make nice with Starfleet command about Tom, leave him to make his own choices, and let Daniel Byrd stay an ensign for years on end. Marry Libby, make babies, rank up. Nothing would have stopped him... except loyalty to his friends and crew.
2. He was very competent. Captain Janeway is a woman with very high expectations. To exceed said expectations, you would have to be an exceptional officer. But in "Twisted", Janeway says he did just that.
3. Despite being unsuccessful in love, he was something of a catch, romantically. Young, good-looking (I assume), loyal, intelligent... and, according to Linnis in "Before and After", a great dad. And in "Lineage", he gave Tom some pretty good marital advice. Only downsides: according to Tom, he snored, and kept his room too hot at night (maybe having someone warm to cuddle with would fix that).
4. He was eminently promotable. In five separate timelines that I know of, he achieved higher rank. In three if them, command rank. He wasn't under Janeway's command in any of those timelines, though... maybe the fan theory is right, and she did secretly have it in for him!