"Fizz's E/Tahm Kench's W is a free Zhonya"
Don't understand why people say that.
Neither of them can save you from delayed or DoT damage.
For example, if your ally is affected by Malzahar's Malefic Visions and Tahm Kench devours him, your ally is still going to die.
Also, devouring a target that has been marked by Zed and dealt enough damage to die will not save him.

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ult ) AND he moves.
Tahm counters Zed not because of mark dodge(that doesn't work, as you said), but because when Zed ults ADC Tahm just eats him so Zed can't do any damage for the mark.