Assassins were updated so their burst is either less reliable, or harder to set up, allowing counterplay. Tanks are getting updated so their damage is less reliable in exchange for more cc. Riot has said that they're fine with mobility, as long as there's enough trade-offs for it, but maybe the issue is that mobility is too easy to use. If dashes and blinks were harder to use, maybe certain champions wouldn't feel so difficult to fight against.
GankedByWindows2 votesI don't think assassins were made so their burst is less reliable, but rather that it isn't instant so there is room for counterplay. The golden example of this being Zed's ultimate, which is why they didn't rework him with the rest of them. For example, they added the delay in LeBlanc's passive going off.
Lauchmelder2 votesMobility in itself isn't the problem.
The problem is giving any champion** too much** mobility while also giving them too much damage.
Smitemuffin2 votesWell, the champions with mobility don't actually have any trade-offs for it. In fact, the super-mobile champs are often S-tier or A-tier in additional areas, especially considering the positions they're being played from. For example:
A-tier waveclear, pick potential and overall laning strength; practically the safest blind pick in mid lane and one of the very few AP champs with sustain or true damage
Strongest AP champ late game and pretty close to the same post-6. He's so powerful I feel guilty for even playing him. Clearly tied with some of the other assassins for S-tier burst
S-tier at cancer, most toxic player group in the game, always the best player on the enemy team regardless of which nexus he spawns at
I don't think I need to cover this one
A-tier waveclear, S-tier burst, S-tier split pusher among mid laners (A-tier split pusher overall) and the only champ that shows you when a target is in kill range of his all-in combo. Surprisingly, actually has the longest basic poke range of all mid laners unless you count a Varus Q (which only outreaches him by 50 units at max). Zed is also the best under-turret farmer in the game. On top of all that, he's an utterly oppressive laner who's perfectly safe to blind pick
S-tier split pusher since his update, S-tier burst, A-tier waveclear as soon as he gets 
A-tier waveclear, S-tier burst, second-largest shield in the game and I think he has the largest AoE stun
This champ has literally never been out of meta or less than an A-tier jungler overall. His blatant universal superiority has been a running joke since his release
S-tier poke, totally uncontested since the Varus nerfs. A-tier sustained DPS, which is a bit surprising for a caster ADC
whenever she's even remotely playable she's totally broken. Only champ in the game that can guarantee baron/dragon secures. Best hard-engage of all Marksmen with the right support, otherwise she's slightly behind Ashe
Obviously there are more and these are just some highlights of the simplest examples. But you get the idea
Eedat4 votesWhat are you going on about lol. Zed does not have good waveclear. Melee range waveclear is not "A-tier". His W has a 22 second cooldown and is also his engage and escape.
I'd go on about the others but your entire post is pure salt and misinformation
RogueWill1 votesSet up shadow in the mid of the minions, spin the throw shuriken; wave is successfully cleared. Among the melee assassins he indeed has the safest waveclear since he doesn't really have to get into range to farm or push the lane. The only other close to it would be Diana with her high AoE and shield to mitigate the damage but her mana cost balance this up.
Eedat1 votesand burn your 22 second escape and engage tool to do half hp to a wave? A W>Q>E combo doesn't even come CLOSE to clearing a wave until you have like 3-4 items
RogueWill1 votesyou only need half a full item to clear it (the cannon will survive but just last hit him under turret). I said it was a safe clearing not perfect, if you clear the wave you usually stay close to the turret to wait fro the CD, you shouldn't have problems waiting for the CD. You usually engage when you have your ult and is independent of the wave. If you want tot clear the wave usually means the jungler is close or you can't reliable trade (maybe teh enemy is ahead) in either case you wouldn't want to engage.
Dominick Destine1 votesTrade offs you say?
Shuyin1781 votes 
Both of their movements (Ezreal blink and Tristana hop) cannot be negated by CC.
I can't tell you how frustrating it is to CC one of them, but they still get out because of it. I have rooted, suppressed, stunned, snared, knock into the air, and hooked both of them hundreds upon hundreds of times over the years and it's just as frustrating every single time watching them just get away from everything AFTER they should be locked down.
GankedByWindows1 votesEzreal's can't because its a blink. You either got him or you didn't. If he starts the cast it will go off.
Tristana is similar, but not the same. It actually is possible to knock Tristana out of her jump, because it is a dash. Watch her jump in slow motion. During the cast time (before she gets off the ground), you can't knock her out of it. If she has jumped and is in the air, you can CC her. Go into a custom with a buddy and thresh hook/flay her out of her jump.
Shuyin1781 votesI've played for 4 years, you aren't explaining anything I don't already know.
GankedByWindows1 votesYou said they both can't be negated. That isn't true for Tristana.
Shuyin1781 votesOn windup they can't, which is what I was talking about.
GankedByWindows1 votesFor Trist that just means you timed it poorly. There is a way to CC her.
