Opposition Agent looks fine to me, honestly, because there are fewer ways to
force someone to tutor.
If you wheel with Hullbreacher out, not only do you get the whole Narset "I draw cards and you don't" thing, but you
also get a bunch of treasures, effectively making the wheel free. Then you have stuff like...
Don't get me wrong - both of them slow down games. But I feel like Hullbreacher is much more likely to lead to one person top-decking while the other player gets to play normal Magic.
Granted, this also depends on cube composition. If Hullbreacher and Opposition Agent are mostly just being used to "steal" a card draw spell or a tutor, Opposition Agent is going to feel way worse, even if you just get a basic land out of the deal. If you're running a lot of incidental card draw, Hullbreacher is going to be way more of a kick to the teeth.
I also think that hating on villain's card draw should be a Red, Green, or White thing. Red or Green because they're Blue's enemy colors, and White because if you aren't going to give it proper card draw, at least let them stop villain from running away with card advantage? I actually would have loved Hullbreacher if it was RW instead of U.