Nimbus Cloak is buggy, at least with Sions ult.

Nimbus Cloak doesn't always act the same when I use it with Sions Ult. Sometimes its effect activates right when he ults, sometimes its effect activates when the ult ends. Its hard to say which I like more, though +100 speed at the start of Sions ult does make it harder for the enemy to dodge at closer range ulting. Conversely, if you miss the ult, nimbus cloak activate after lets you chase after the missed enemy fair well.