User blog comment:DoctorAsawa/Mute, the Silent One/@comment-76.94.97.239-20130326155823/@comment-5514182-20130327232042

First of all, what would Karthus do in the 1 second after he ults? He's on the other side of the map.

Second, the passive activates on 10% of max health, so there is a huge penalty.

Third, he has a 150% AP nuke if it hits a silenced champion, I believe that's considered burst.