DDoS'ing mining power would be more effective. You don't need to outcompete miners, just ensure they are slowed in their ability to push blocks to the network.
I'm not talking about effectiveness. I also think it's more effective to attack PoW than PoS, but my point is there may be cases where the attacker's top priority is not effectiveness.
Traceability is one such example, but there can be many other cases. Proof of work is pretty straight forward because the vulnerabilities are mostly technical issues, whereas proof of stake I think can have a lot of social vulnerabilities.
I bring this up because I feel like every discussion about blockchain security seems to mention "effectiveness" but the world is not perfect and there are plenty of cases where certain parties act in unintuitive manners.
