I had wondered about this as well.
I just want to clarify something about scathing tirade: Each success = 1 more target (so 3 successes, 3 targets getting 1 strain each). Each advantage is 1 more strain, but is it 1 more strain to 1 target, or 1 more strain to all targets?
Using the above example, would success 3 advantage 3 be 2 strain to 3 targets, or 4 strain to 3 targets?
Also, can you convert successes into advantage if you have more successes than targets?