Jhin's ult gets a small refund for every shot that he doesn't fire while ulting. Meanwhile, Xerath's ult goes on full CD unless he doesn't fire a single shot. As far as I'm aware, their ults aren't dissimilar enough that one should be able to partially refund the ult CD and one can't.
Aside from the fact that those abilities belong to champions with vastly different playstyles, I would argue they are distinct enough to warrant a unique refund mechanic on each one.
Jhin's Curtain Call is quite restrictive and unforgiving, due to the fact that it is...
Given that, I find adequate that Jhin gets a cooldown refund per unfired bullet, as he will find situations in which remaining on firing stance is just detrimental (full HP Tank blocking the shots, the teamfight moves out of his killzone).
Meanwhile, Xerath's Rite of the Arcane has the following advantages:
Rite of the Arcane is just way more flexible than Curtain Call; giving Xerath a partial refund mechanic on it would allow him to extert even more pressure on the map: He would be able to rain arcane fire over the enemy team from a more than a screen away, safe from retaliation and knowing that if the enemy retreats, he can cancel the ultimate to have it available earlier instead of commiting its significant cooldown. It would also make him very strong at minion wave control; something that made Ziggs (another Artillery Mage) so hard to balance in his time.
