I do agree with RR that Immortal as a stand alone card may seem unbalanced, but I don't think it is due to the way it is used(mainly, in mono aether).
Mono Aether already does have a pretty good defense, I don't think it needs to be any faster.
Immortal is still a valuable card since it's the best option to make a mono aether deck faster, if you use phase spider instead, you're giving away one of aether's advantages, immunity to cc, especially antimatter.
Phase dragon is more card effective? sure, but how often can you play a phase dragon 2~3rd turn and how often can you play an immortal?
I think those things should be taken in consideration and not only the dmg/cost ratio