yea its broken.
someone in the hive workshop dissected the mission. essentially the AI is choked by the changed ruleset from roc to tft.
in the original game demon units didnt cost supply, in reforged they do. the AI is supply capped all the time hence being unable to revive the heroes and boasting an overall smaller army - roughly half the size.
on the flipside the TFT ruleset makes units alot cheaper for the player. initial gold cost is vastly reduced and upkeep kicks in later.