that just goes to show how much you dont know about the champs you dont play.
karthus's R isnt his powerhouse and anyone mad about his R is making that argument solely because they died to a karthus's R. most of his damage comes from his Q and his kit is defined by his passive. If you die to a karthus R with

in the game... you should honestly quit for a day and take a break to explore other alternatives for enjoyment.
sorakas R is a heal to all of her allies that can swing the tides in her team's favor. that being said sorakas kit is terrible because she's a support. all supports have terrible kits and their R is USUALLY their biggest asset and sometimes their ONLY asset.
taric is a support his ultimate grants invulnerability without using stasis for roughly 3 seconds. so yes if you're going to use his ultimate as an argument about his kit all supports are overpowered. lulu especially.
malzahar's R is negated by several items. not just QSS. in fact if it werent for his minions malzahar would be terrible
illaoi's R summons 1 tentacle per enemy champion hit for a max of 6 tentacles. Her R requires her to be standing beside the opponent. im pretty sure if you're going to make an argument about illaoi you'd be better off talking about her vessel mode or her health regen since she's a tank (dont correct me with juggernaut or berserker or any of the other development team nicknames because i use the in-game 6 categories to define my champs i.e. tank, assassin, mage, fighter, support, marksman) without mobility abilities.
if a game lasts 30 minutes and an ultimate can be used every 60 seconds and you reach rank 6 at 10 minutes in (i dont know when most players reach rank 6 im guessing) then 20 minutes means that you can use your ultimate 20 times in one game. most champion do not have 60 second cooldowns for their ultimates. some do not even have ultimates (elise, nidalee, etc.) so if a character strengths are defined by solely their ultimate it means that they're poorly designed and will never be able to compete. if you have to talk about a champions ultimate then you're basing your argument on shaky ground to begin with. in the rare case that a champion has to rely 99.9% on their ultimate then they're not a truly viable champion or they're broken and should soon be patched.
besides all of that i said USUALLY so even if you and everyone else disagree with my reasoning the fact that i said usually and then you go on to point out specific situations where one might use ultimates and one might not use ultimates to make an argument about a champion just further emphasized my point that you dont always use ultimates which is why i wasnt using the ultimate for the basis of my argument.