You managed to keep Yasuo out of this thread, good job, that will keep me and the rest of the Yasuo mains at bay.
Mobility is kind of an issue, but not for the reason that people think. When they think of mobility, they think of damage as mobility is normally associated with Assassins. Mobility there requires another stat to give up a bit, in the assassin's case, it's survivability and defense.
The MOMENT where mobility exists on a champ with some defense built into their kit, it begins to get to the point of imbalance.
Flash is meant to be a spell where, when all other mobility you have fails, you can use it for instantaneous transportation. This is useful on some champs who have no mobility as it provides a window where you can get onto them or get away easier.
On mobility champs, this is a much different. WIth a champ with mobility, this compounds even more mobility into their kit. This can be frustrating to deal with, but it serves a purpose. Flash provides instantaneous mobility when the champs mobility runs out or the situation calls for it.
This can be difficult to deal with as the only way to kill a champ with mobility is to lock them down. When mobility runs into defense like Camille or Yasuo passive, then killing them can be hard to do and can lead to mildly toxic kits.
Defense can also come in three forms. Raw stats, defensive kits and scalings, and build paths. People with raw tank stats end up being immobile tanks. Some champs, like Yi and Yasuo, have defense built into their kits, but don't have the raw stats or the build path that give them defense. Then you get to the toxic part. When a champ's buildpath gets to the point where it is defensive and still gives them damage, then it sucks.
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who have huge mobility, almost 
and 
levels and it just does not seem terribly fair to other champions.