About Rengar's Ultimate
We know that Rengar is extremely feast or famine in his design, he either obliterates people while under ult and catching them out, or he's sub-par at best. A lot of this is due to the way his Ult is designed, while there is a (!) mark suggesting that this is sufficient warning in most solo queue play isn't valid. Therefore he often has to be balanced around it not even existing.
What do Nocturne, Rek-Sai, and Sion have in common, that Rengar doesn't? A Global Announcement whenever they ult. Having a global roar when he goes on the prowl, having the current range indicator be (!!), and doubling the range of what is now labeled as (!) indicator, or hell even implementing only one of those two changes would give sufficient warning.
tl:dr: The current (!) warning is nigh meaningless. It should serve 'some' purpose.