Sion's ult has a global sound because it gives him too much global pressure if you can't hear it.

you get a ! if he's close to you :3

If Rengar's Ult warning was global that would be a huge buff for him since you'd have no idea if he's close...

If you wanted to nerf him, you'd want to decrease his speed and/or slightly increase the ! indicator range.

Ugh, another rengar thread. I feel rickrolled.