Okay, let us start. No, they do not have good clearing tools. When i refer to good clearing tools i refer to tools like ziggz q or lux e. Which are actually good clearing and farm tools. But i will debunk you on case per case basis.
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Akali - Correct. However, she is highly overtuned and in an op state.
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Diana - Melle, only real clear ability is also her only intiation ability, high mana cost, low range.
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Ekko - Correct. Highly overtuned as a champion and unbalanced.
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Kassadin - Melle, the only clear ability is incredibly low damage, requires set up, low range and is his only cc ability for fighting.
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Katarina - Correct. However, her assasination potential is lower than other champions and requires set up, as such she sacrifices her assasination ability for increased waveclear.
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Kayn - Correct, however, again assasination potential is lower than other champions because of the waveclear, clear sacrifice for waveclear.
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Kha - Melle, only real clear ability is his w, high mana cost, incredibly low range and low damage.
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LeBlanc - Correct, however, assasination potential is lower and more spread out because of the ability to have waveclear, clear sacrifice for waveclear.
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Nidalee - Not an assasin.
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Rengar - Not an assasin.
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Riven - Not an assasin.
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Yasuo - Not an assasin.
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Zed - The only waveclear he has puts his mobility and assasination potential out of commision, is high cooldown and comparatively low damage, makes him vulnerable.
You mistakenly think that all waveclear is GOOD waveclear, it is not.
Two of the assasins you listed are in an unbalanced state.
Other assasins sacrifice their assasination potential (As in their assasination tools are worse than others have or require a lot more set up) to have extra waveclear.
The third group sacrifices their initiation and damage tools for waveclear which makes them vulnerable to any attack for quite a long ammount of time, this also does not even fully clear a wave.
And the fourth group has very shitty waveclear that barely qualifies as that, it makes them vulnerable, is low damage, high mana cost and all in all, bad.
Compare any of these "waveclears" to any waveclear that mages have and you will see how shit these waveclears are or in cases of kat/Kayn, how much they have to sacrifice in assasination potential for them to clear anything.