I can't speak for everyone, but personally I just dislike the lack of resolution.
Every other Origin character, even if the ending isn't particularly happy, still gets a resolution to their story. Asterion is free of Cazador, Gale has made peace with Mystra (or died trying), Lae'zel has turned her back on Vlaakith, Shadowheart saved (or lost) her parents, Wyll saved (or lost) his father. While many of these may not be a happily ever after, some of them are, but importantly, they're free of of the shackles they started the game with, or at least made their peace with them.
Karlach however gets no such closure. Either she resolves to return to Avernus, the one place she stated numerous times she'd rather die than return to, or dies a miserable death. Narratively it's unsatisfying given that her core character arc (if there even is one) is unresolved. If this is a choice based game, where my impact on the story is necessary for it to progress, why are the options available to Karlach the same as she started the game with? Either she dies, returns to Avernus, or becomes an illithid.
