My issues are, like Elian said, his edgy behavior, which reaaaaaaaally stands out among the rest of the cast, even Adachi. He sounded super cartoony and dumb which I guess works well considering what Yu eventually became in each following appearance he's had as a character after Arena, which I guess was the point with Adachi just tagging along with him while secretly laughing at him for the brat he is, which makes me wonder if his behavior was intentionally that over the top to look stupid or if they just realized it after the fact.
Whatever the case, most of P4 spin offs don't really treat the characters in the best light and Arc is known for over the top crap, but it's just kind of... it feels off, Sho feels super off, similar to how people see Marie in the context of P4G after playing the og
In short, people just really dislike him for being a brat with stereotypical japanese bad guy traits like laughing like a maniac, loving your own sadism and also being loud. In short, edgy.
Remember most people came into Arena/Ultimax AFTER either playing the original PS2 Persona 4 or P4G which came out really close to the first Arena