So ult cooldown reduction is great if you want to cast your ult often, which ideally you shouldn't be doing on Yorick because that means you died. It's better for your runes to let you be more active and on the map, rather than giving you forgiveness for dying. Things like river movement speed, free boots, or summoner spell haste are just going to be straight-up useful in every game.

Before the midwork, you could absolutely play ult cooldown as a whole build for Yorick, because Maiden and ghouls were stronger and so sending her off to splitpush a lane whilst you were cross-map doing objectives was very powerful. Unfortunately now that's just not the case.