I thought Harry was a good character at the beginning when he was established as this sort of Starfleet nerd with somebody to impress. They started to use him as their 'suffer' character the way DS9 used O'Brien, and I liked him in Emanations and Non Sequitur.
Unfortunately the Voyager writers had no idea how to make a character interesting without a gimmick to exploit so he either kept getting 'sidekicked' or he got terrible episodes like Favorite Son that had nothing to do with his character.
Harry had a good premise, just like the other characters on the show. The problem was that the writers made him the "whipping boy" of the series, the guy who mostly is beaten up and/or the young, unexperienced kid who everybody feels sorry for. He's actually better in some of the books where he's doing something useful and where his computer skills are developed and used. Unfortunately, he has the bad luck of ending up in sickbay in many of them too, badly wounded and in some cases close to death.
But don't forget what the man, or more correctly, The Clown, said:
The spoil goes to the winner and you are the spoil, Harry!
By the way, am I one of the few who actually like "Favorite Son"? Harry ending up in a future version of Dracula's castle and manage to get out of there. Definitely better than the rather boring "Emanations".