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Yelena is for NOW!!! She will fall off so hard lol. However, she'll still be relevant on AW and endgame content defense simply because you'd have to deal with the reversal but for BGs as soon as they start releasing more mutants with daunted, she'll die just like that because there's barely anything you have to play around with her. Her biggest threat is that damage cap which (FOR BGS) can be avoidable by giving a mutant good damage and daunted. I haven't read her kit but from the deep dive I saw her combo meter going down as well so it seems Cassandra even has a way of stopping combos.

Bullseye on the other hand is still wrecking and with power gain metas, she's alot more of an issue. Maybe my opinion will change once I get ntw and other Bullseye counters but for now, im still scared of him.

I think you hit the nail on the head, i suspect Yelena will eventually undergo the Peni Parker effect where people took her out of decks cause of how prevalent Galan, Hulkling were. Given her kit, a lot of the new release or rework mutants will have daunted or a way of dealing with the reversal thing. They already released Casandra so its just a matter of time. Plus Her offensive use is very limited too (worse than ****)