Having Mercy be non meta is okay

No it does not.

We’re saying that A =/> R (A does not imply R, R being rework)
That is definitely not equivalent to say | If X =/= A
                                                                  |X => R
Logic.

100% pickrate is probably a bit hyperbolic.
And seriously, 0 counters ?
Hold on:

Citation needed.

Almost everything had to be nerfed in her kit in order to keep E-rez.
One of her oldest buffs, the one who made her much more viable, has been reverted.
Bad design.

Yeah, no counters…