Clarification on Riven

Because she has no sustain and is melee.

Now can someone tell me why Gnar has Range, mobility, burst, CC and a better version of shyvana's ult at level 1.

because she is melee, has no sustain, has to build full damage to be useful, and has to invest a lot into CDR items.

also her gapclosers each cost her damage if she uses them without hitting someone.

If you would like we could compare her to lee sin who has more mobility on his Q and W than she has in her entire kit in addition to a %health execute allowing him to build tanky while still doing tons of damage.

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Someone can tell me Why Riven, with 4 dash with no cost and low cd, has 345 of base m speed?? And Why she has 50 sec on ult rank 3?

I can't speak for the others, but my experience on Riven has been that she starts slow during the laning phase and if you don't get good items quickly she can't burst someone, which means she often gets smashed when the retaliation comes. She is also a champ that requires practice to get the timing down perfectly. If you don't time everything right, you either waste time, or you miss on autoattacks you cancel because you are spamming abilities a little too quickly.

Riven deals burst damage. She's not going to survive going into close combat with any tank or even most of the other fighters in the game. But she can take down mages in a hurry with her abilities. Look for ways to counter her. Her attack is a dash, so make sure to have minions in the way so she can't hit you directly and you have time to move to the side once she charges in. In a team fight, have your support CC her as she has no innate resistance to CC or see if your assassin can gank her before she gets started. Riven can't take damage. She's not as squishy as most mages, but she can't take hits.