Speaking OOC here for a moment, Ohya's Confidant was enjoyable imo, I really wanted to help her with her situation, and her abilities aren't useless to me, sure if you do well enough they aren't needed but you can say that about most Confidant abilities, so....? Ohya's skills were a life saver for me in a few instances I fucked up, so they weren't useless to me.
In my opinion the most useless Confidants in terms of skills were Iwai, because by the time he's unlocked it's a bitch to make his skills useful at that point, and Shinya, for a similar reason. Sure it can potentially be broken but it's easier for me to just ignore the Down Shot and save ammo for when large groups are weak to guns.