You only play morde if other lanes can make up for his lack of cc and mid game. You typically have a mid with a strong mid game and/or good cc. Morde lacks both. So you need somebody like Naut, Mumu, Zac, etc. etc. in the jungle who has good, easy to land cc, and a top lane with a decent engage, like Shen, Rumble etc. etc. It's also preferable if you have a strong cc support like Leona, Sona, Nami etc. etc.
Basically, you need cc to back up morde. Otherwise morde will accomplish virtually nothing.
In addition, morde gets shit on at top at all points of laning. If there's absolutely one thing you should never do, it's morde top. He has a weak early phase with no real sustain, a weak ult in early levels, and is forced to push hard to maintain his shield, which is the only way he'll keep himself in lane, which opens you up to hard ganks.
And of course, his ghost is strong. It takes whoever is killed, and gifts them bonus ad and ap, plus himself bonus ad and ap based on the ghost's stats. It turns a 5v5 into a 4v6 if it happens, and it is terrifying to fight against. However, he should basically never get in range to use it on anyone of importance unless, as mentioned before, cc is heavy on your team.
tl;dr: never go top morde, have cc to back up morde in general.