Mal'Ganis' kit looks like shit, really. His stun is pretty bad. It's literally Riven from League of Legends, but you don't lanefight nearly the same way as you do in LoL, which is where Riven's Q is mainly helpful. Just imagine trying to position yourself correctly to smack people in a team fight to apply your stun.
His Terrorblade ult is stupid.
He's another weird "support" tank where he needs someone else to CC for him because HotS is a mess where most heroes who can't CC are inherently gimped and need help.