Why? It’s simple. They’re just plain underwhelming. Before the movie came out, these five got featured in so many toys, teasers, covers, and other promotional material, even the intro sequence to Friendship Games portrays them as well-known enemies of the Mane 6, each member having their own rival counterpart. They were constantly advertised as if they were going to be important characters and a finally canon antagonistic Mane 6 with interesting polar opposite personalities, right? No.
Seriously, I can’t even name two personality traits about any of them. And yet already these five have a TON of fanart, shipping, headcanons, fanon personalities, etc… and I understand why. They have the background ponies appeal. Secondary characters that have minor roles and little (if no) personality, yet fans just love their potential and they get popular pretty much only through fan portrayals.
Do I “hate” these characters? Not necessarily. Hell the main reason I defend characters like Flash Sentry so much is that he’s SUPPOSED to be a side/minor character, yet people treat him like he needs just as much development as the main characters for some reason. I would have probably been more accepting the Shadowbolts if their roles were advertised less misleadingly.
Still, this is just my opinion. Love or hate these five as much as you want, everything I said here comes purely from my personal taste.