I think that Nether Portal is simply ridiculously restrictive. I tossed couple of remarks about that in official forums - my take is that Blizzard threw just about every restriction they could at it - high cost, long CD, long cast time AND ramping up effect that requires you to actively cast cast cast and god forbid you need to move there AND the effect is a bunch of guardians with all the inconveniences of it.
IMO, there is no reason for it to not be either free OR instant cast and then having its payload spawn passively at some rate like 1 demon every 2 seconds.
Of course they could buff it through the roof, but IMO at least they need to ease up some of the restrictions too, it's just way too prohibitive. The idea itself is sound, but seriously if one would ask me what kind of other penalty should be put on this thing, I'd shrug because they already pretty much put every last thing they had there on it.