Karthus has to channel 3 seconds on his ult

{quoted}

karthus ulti is here to warn you that you will take damage

you should also be warned that soraka is healing her team mate

Well Karthus' involves your health bar, that's why you get time to try and mitigate and 600+ dmg nuke. Soraka doesn't directly hit you with an ult, and also since it's a heal it's meant to be used in a pinch.