I left out the scaling on purpose because Darius doesn't build that much AD usually. He gets BC, maybe one other items then fight tank items, Spirit or Deadman's. Let's say he does have BC and Titanic Hydra, that's 80 bonus attack damage his other items are boots and a deadman's at 15 right? So 400, if he gets all the stacks, plus 120, so 520, plus passive damage in that time, so 5 stacks, at 15 is 135 plus 150% bAD, so 250. Of course not all that 250 will be used if he actually ults. :P Plus his passive is physical damage so you can get armour against it.
Now Pyke, at 15, he can't build HP so let's say he 'supporting' and has a Remnant of Ascent which is 350 HP, that's 25 bAD, then he had Duskblade and Youmuu's, so another 110 bAD, 135 total. At 15 that's 525 plus 189, 714 damage.... meaning anyone below 700 HP is dead. No stacking up there, run in with W, press R before people see you and win. Darius can't do that. Imagine if he could, if he does it's only 200 plus 75% bAD which isn't much.
But I do like people talking about this, it shows the Ult has contention from both sides. We already have compiled PBE videos of the examples above but as we know, Riot doesn't use the PBE for balancing but for bug testing (Remember AP Galio? ). We won't see what happens until he's released... and then gets nerfed. :P