After being in a good number of games both with and against Senna, I feel like her ult is way faster than a global ult should be. Other champions like Jinx or Ashe or Ezreal have relatively slow-moving ults that take a while to get across the map, and it takes a lot of positioning and luck to connect it long-range, especially because the hitboxes are relatively small. But Senna's ult is at least like 3 times as fast as other global ults, and it feels like twice the hitbox size. I've seen multiple times where Sennas have used her ult to steal a kill way on the other side of the map, pretty much instantly. In one game I had, Senna completely trolled our Nasus, ulting nearly every kill that he tried to secure all the way from botlane and once from spawn; there's no way that an Ashe or a Jinx could do that. It really shouldn't be possible, and I feel like it just enables trolling and toxicity.
This could be easily fixed I think by making it the same speed or only slightly faster than other global ults. Another (probably more controversial) way is by making it have the same range as a Lux ult or something, but definitely not global.