Soraka needs her own health bar to play so on every karthus ult you can't do anything. Something weird like banshees is too late to matter, you could try redemption or crown rush but still. And soraka R heals multiple people but soraka W is single target so it's not great when multiple people are low and karthus is channeling R.

Sona W shields and heals herself every time automatically, so she's much less threatened by the karthus ult accidentally cleaving her to death. If he ults before fights she can get a lot of moonstone healing (liandry's=in combat) and negate the poke. If he channels R at the end of a fight she has enough sustained healing to continuously heal people even after however many seconds of fighting (as long as not oom), plus w can shield multiple people. Also seraphs is a viable option and self shields.