I think one of the issues with Yorick's ghoul management is that he starts out with one way of summoning ghouls, then almost completely drops it in "real" combat in favor of another, so he never puts his whole power to the best use. In lane, Yorick needs to summon ghouls slowly, because otherwise he'd revert to pre-VGU Yorick status and be a pain to play against, but in actual combat he needs a means of summoning a bunch of ghouls instantly, which his ult currently provides. Once he reaches level 6, he basically has no reason to summon goals the basic way unless he's splitpushing.
What I think could help Yorick is if the model for his Q summoning were tweaked a little: if Yorick could use Q to summon ghouls around himself whenever, and recharged through minion/champion deaths instead of relying on static lane-based objectives, he'd be able to have ghouls when he needs them at all stages of the game, while still giving his opponent breathing room in lane. As an example:
Q - Awakening: Yorick summons four Mist Walkers to his side. Has an X-second (long, probably static) cooldown, reduced by Y (a small amount) every time a non-champion enemy dies near him, and Z (a quarter of the cooldown) every time an enemy champion dies nearby or he kills an enemy with Last Rites. While Awakening is on cooldown, Yorick gains access to Last Rites instead.
(In fact, you could probably make Yorick passively auto-summon four ghouls and/or refresh his roster periodically, and have the cooldown get reduced by the above, without having to keep it as an active)
Re: his ult specifically, I think he can do without the bonus on-hit effect. As it stands, the Maiden of the Mist is meant to be a fairly independent minion whose strength lies in her summoning and durability, and who should be pretty good as both a splitpusher and an in-combat aid. Giving Yorick a bonus for autoattacking her target, i.e. tying some of his power budget into an effect he will not use all that frequently, is one of the reasons he feels weaker than he truly is. Making Yorick and the Maiden more independent from each other would allow him to be a strong splitpusher, while still having good options in teamfights.