There is a fundamental difference between Skarner's and Darius's Ultimate.
The reason why Skarner's ultimate was nerfed in that sense was because if you got caught out of position, Skarner could drag you into his team to just die from full hp. You could go from 100% to 0% and there was no real escape from this apart from Quicksilver Sash. This could happen under towers, in team fights, in ganks and given Skarner can carry flash too, he could flash in, grab you and leave with you, wherein you died - again, from 100% to 0%. This is why the nerf happened.
Darius's ultimate cannot kill you from 100% to 0%. It is designed not to - it needs him to do damage beforehand or for you to already have been low (presumably by fighting) and in that sense, your ability to survive comes from your cc and stuff from before he uses his ultimate, not once he starts it.
This is also why the leap into the air is important - if the fight was close enough, you might be able to kill him during the half second where he does no damage and walk away a winner. It is rare but it happens.
Constantly comparing Skarner's Ultimate to Darius's Ultimate does not work, Skarner's ultimate before was WAY better because, with proper play, it was essentially Darius's Ultimate but with more damage.