I think the difference is that most people didn't grow up seeing safe PG and PG-13 Invincible cartoons and comics and seeing characters they're attached to since they were kids being dismembered and depressed fucks with them.
Invincible showed people early on what it was going to be about and what to expect and it wasn't geared towards kids. So seeing Invincible and others suffer like that isn't as jarring.
I recall the reaction to Marvel zombies (the comic by Kirkman) made people upset too. Seeing your favorite childhood heroessuffer and act like unheroic cannibals is jarring. Still that was nowhere as bad as what Warren Ellis did with the Marvels parody RUIN, I wonder who fucking approved that shit.
Oh and there's also that "NGHHH Gwen" panel from the later Marvel Zombies mini, you know the one with Sandman, who wanted that?