Morde is like a lot of tanks though in that he's quite poor early game. His damage isn't that amazing either unless you gear up appropriately, but his skills all add to his shield, and that's why he can appear so tough, especially when he's wading through mobs, as he's basically getting hp from the shrapnel leech.
He's not as unkillable as some of the other characters can be (Trynd with his ulti played well can be downright nasty as an example and I've seen him decimate an entire team by himself)
Mord is immense early game.
He has almost unparalleled lane dominance. His skills are mana-less and regenerate a shield which generally negates pretty much all harassment from his opponents. He can clear creeps very easily without falling foul of harassment or being zoned out unless he's in a 1v2 lane (and even then it takes smart creep management to zone him out).
He falls down when focused mid-late game. If your team has a good balance and takes out the opposition carries effectively Mord isn't a problem, but if your team isn't switched on and one of them gets caught away from the group and dies early (particularly an AD carry), then you're going to be in for a much tougher time.
With a full load of items he is a force to be reckoned with, but there is a reason no high level players bother with him. He's not as good as a proper ranged carry, has minimal mobility and zero CC.
Overall Mord is a great pubstomper, but he absolutely should be played in a solo lane.