Again, 3 second delay after a .5 second casting delay, and again, breakable by distance. Also, 625 range. If Morgana is in the center of your team, ulting them all, then simply by walking in opposite directions, Morgana would only be able to keep one person inside of it, and only if that person has no movement abilities. If they get out, the damage becomes pretty minor, and it doesn't apply cc at all.
That's enough time for Taric to cast his ult and then wait for it to land. That's enough time for majority of champions to unleash a full rotation, most of which can kill Morgana. This is why Morgana very, very frequently rushes Zhonya's hourglass, but that also makes it easier to walk away from her ult. To even get close enough for Morgana to use ult+Zhonya's effectively, she often needs to flash in, and even then, again, anyone with movement abilities or flash can get out of it. Even if all goes according to Morgana's plan, she then has 1.5 seconds to run away before she's immediately deleted by the team she has put herself in the middle of. Hell, she might not even have that long, because there are almost certainly going to be part of that team who could dash or flash away from her ult, who is now able to turn around and make her pay. If Morgana's team can't follow up immediately, Morgana is going to have a lot of trouble, since she no longer has flash to escape.
The base damage on Morgana's ult is pretty good if you get both parts, but the longer the game goes on, the harder it is to do so. This is why the scaling is so high; if it did no damage late in the game, it would become pointless putting yourself in danger to tag more enemies in its radius.
With 30 AP, (Doran's+AP Quints) triggering 3 stacks on Brand's passive does 21% max health damage, in addition to 15% max health damage to anyone standing too close to them. Later in the game, he easily outscales Morgana's damage output and doesn't have to wait three seconds to do it. It's allowed to hit hard because Q+W while great and all, is too weak alone to let her perform as a Mage in midlane.
If you want to compare it to Lux ult, fine. Lux ult has the same base damage as both parts of Morgana's ult combined, can be cast from more than 5 times farther away, can be cast more than three times faster, Both applies and detonates Lux' Illumination passive (Doing 70+20% at level 6 potentially twice) and scales way better into the late-game. It's even aoe, just much narrower. By no stretch of the imagination does Morgana's ult do "double" what Lux' ult does. They're apples and oranges to begin with.
