It's not really that broken. Yes there's no cooldown but the damage is very low unless you pair it with Draugrkin. A build running Shared Pain + Draugrkin has many drawbacks, especially when you're fighting players that don't use DoTs (e.g. Mag Sorcs)

The best setup is obviously against a DoT DK where Shared Pain can be 30%+ of your damage. But those 1v1 matchups are rare unless you're only dueling. In Cyro/IC/BGs, there's much better setups.