That’s because the Ballista uses a bow attack, so the 45% of your melee damage will do precisely nothing to the Ballista’s bow attack. I’m not sure I’d call it “needlessly pigeonholing” though since the only way to get bow damage is by equipping a bow. Would you expect to be able to do lots of damage with Detonating Arrow if you had a melee weapon equipped?
You can argue that the node should state that it applies player bow damage.
Going from up to 200% crit multi to up to 135% crit multi is a reduction of almost 1/3, that’s a fairly sizeable nerf. Though given the base is 200%, it’s “only” ~16% in real terms (less so if you have other sources of minion crit multi & I can’t see any of those on the Necro passive tree.