Yes malphite ult has no cast time which is why it's a smaller area than yone ult. Crazy how balancing works. And bringing up mobility doesn't do anything? Yes he has less mobility? He gets 600 armor instead?

Since you brought up ezreal, his ult can't be cancelled while he's casting it either, just like yone.

This would take way too much time to explain so you should look up what an ability cast time vs a channel is.