That just means he turns the fight into a 4v4 and when he dies, the fed carry is back to help his team. And if your problem is that the carry's team might lose the 4v4, well let me tell you something: the current state of the game of one hyper carry being able to destroy the entire enemy team in teamfights isn't balanced, if his team can't win or at least stall a 4v4 then they don't really deserve the win, the only one who might deserve to win is the carry (something which is also often debatable since there are some champs who don't have much counter play and reward the player too much for a small lead). If something, the problem with the ult is ulting champs like Soraka, who is easy to 1v1. Ulting her will deny her heals while you also get a higher chance for a kill, however it's not like his new kit allows him to just automatically catch the Soraka, she can play around him, stalling (and... playing well), it has counter play (also, I don't know, but her ult might still be able to heal her teamates while in Morde's ult).