I don't want to have to sacrifice Ravenous Hunter, Ingenious Hunter, or Relentless Hunter for Ultimate Hat, a rune that could previously be used by sacrificing mana. Nimbus Cloak is gonna make Kennen/Wukong/Yi super obnoxious and impossible to get away from their ultimates, in which the counterplay TO Kennen and Wukong is exactly to dodge their ultimates....
In addition, this basically is Riot saying "We balanced every mid lane mage with the idea that they're ALL taking manaflow band, so we're just not gonna give them other choices." The ONLY mid lane mage who MAY use Nimbus Cloak is Ahri, but she'll probably still relegate to manaflow band because mana changes and no one wants to use biscuits. Nimbus Cloak is quite literally for everyone but mid lane mages. Vayne, Kennen, Wukong, Yi, Akali, Diana(I consider her an assassin, she's considered a diver, so not a mage, still would probly take mana flow band though), Fiora, Kassadin, Khazix, Mordekaiser, Nasus, Singed, Yorick, Mundo, POSSIBLY AHRI, and Zed are the most likely possible examples of Nimbus Cloak. This is just further forcing manaflow band to be taken by all mid lane mages.
Also everyone previously said Ultimate Hat was garbage, so how is this considered to snowball mid laners? It's a 100% nerf to ultimate hat. You now are forced to get kills to stack it, when previously you could just use your ultimate, AND you still get the exact same cooldown reduction from it. On top of that you have to sacrifice SEVERAL OTHER good options to even take it. Let's not forget most mid lane mages aren't gonna take anything else from the domination tree except for eyeball collection most likely.