He isn't underrated. But he isn't viable for a ton of fights. In regular bg fights where opponent hardly gains any buffs, he can't even finish them off, it takes around a minute to take down champs with high amount of buffs.
Exactly. If you try to nullify his indestructible buff, it turns into a passive. Morningstar’s SP2 nullifies all buffs on the opponent and punishes them for every one. This is where Mojo shines. He can nullify with his SP1, but his anti life field reduces the duration of all buffs by 90% so they practically last nothing. And he degenerates them every time a buff ends or gets nullified.