Bottom line is they just copy-pasted the Gem Dragons into the previous Mega-Boss teams with literally no other changes to team composition. For some teams this actually works out fine (Sapphirax, Amethialas), for some teams it doesn’t (Rubirath) and for some it COMPLETELY MISSES THE WHOLE POINT OF THE TEAM TO BEGIN WITH (Garnetaerlin).
This being said, Rubirath today actually did give me a little trouble for once, but it wasn’t the dragon itself, it was Gorgotha constantly exploding the board and charging all its friends simultaneously (Rubirath is otherwise Mana Locked behind Gorgotha and Troll).
Note that if you just swap out Dark Troll for Flame Troll, now Green Slime’s purple mana goes to the Troll who in turn generates Red Mana for the Gem Dragon. Then the boss team works! (Also note how both Rubirath and Amethialas feature the tag-team of Gorgotha and Dark Troll, more reason to swap out the Troll).