The following contains major spoilers for My Hero Academia Chapter 364, "Why We Wield Power", by Horikoshi Kohei, Caleb Cook and John Hunt, available in English from Viz Media.
Katsuki Bakugo's death was one of My Hero Academia's most shocking moments yet. The young hero had only begun to shine in the battle against Shigaraki Tomura when the villain suddenly directed a powerful punch towards his heart, completely destroying it. It was a seemingly irreversible injury, until Edgeshot claimed to be able to sacrifice his life for Bakugo's sake. Unfortunately, the Pro-Hero's selfless act was not well received by a section of My Hero Academia fans.
Bakugo's injuries were described as fatal. In Best Jeanist's words, his heart was ravaged, and he was left without a pulse. His death seemed so definite that he was extensively mourned by fans online, many of whom refused to accept his death and eagerly anticipated a revival. Apparently, Edgeshot felt similarly in his refusal to accept Bakugo's death, claimed to be able to use his Quirk to fill in the tattered remains of Bakugo's heart at the cost of his own life.
Edgeshot's Quirk is Foldabody, the ability to stretch and compress his body into an impossibly thin material. It's the perfect Quirk for a ninja themed hero like himself, as Edgeshot can use it for discreet reconnaissance, slip through the smallest of cracks and even to restrain opponents. The speed at which Edgeshot is able to transform into string makes his piercing attacks impossible to evade for regular villains, and at his thinnest Edgeshot is entirely invisible to the naked eye.
A bulk of the negative reception of Edgeshot's sacrifice in MHA 364 has to do with the fact that Foldabody's specifications have never hinted at an ability to replace living organs. He's been shown to be capable of using Foldabody to enter the bodies of villains in order to knock them out, but that process is nowhere as surgical as what he's attempting now. His sudden declaration that he could replace Bakugo's heart stretched the limits of his Quirk's capability a little too much for fans to accept in good faith.
Also, Edgeshot sacrificing his life for Bakugo does make sense from a heroic perspective, but because the duo have never shared any kind of relationship, the tradeoff seems hollow and contrived. Bakugo and Edgeshot have never so much as exchanged pleasantries with each other and yet, Edgeshot is willing to sacrifice his life for Bakugo's sake without a second thought. Bakugo's worth in the fight against Shigaraki is undeniable; he remains the sole hero to have inflicted a permanent injury against the villain. It's just that Edgeshot's sacrifice would've been that much more legitimate if the duo had a preexisting relationship.
Bakugo was a very beloved character. He frequently topped My Hero Academia popularity polls and his death rocked several fans to the core. That's why having it seemingly reversed after only three chapters cheapens the emotional impact out had in the first place. What was previously an excellent send off to an iconic character is now being regarded by many as an ill-conceived attempt at shock value. If Bakugo does somehow survive, he'll need a more convincing argument for why and how he did than Chapter 364 provided.