The biggest problem with Nasus is how long it takes him to be powerful enough to do his job. In an ideal situation, you can farm to fairly high Q levels by twenty minutes, but that's not going to happen in most lanes, so you usually have to wait till at least thirty to be able to do your job well.
Before then, it's essentially farming, trying not to die, and occasionally teleporting in with the hope that you can take some aggro off your teammates and get Wither on someone that matters.
He's really not bad, but he'll never be a great option for solo queue all the time. There are definitely lanes and comps where he can do very well, but there aren't a lot of them. (I can say, however, that if Nasus gets ahead enough and uses Teleport aggressively to stay ahead - probably because his team doesn't need the help - he can easily 2v1 ganks when they come in if he's past level six. It's actually quite fun.)
As someone who doesn't play Nassus top, but has seen a lot of Nassus tops: Yes he's viable but if you play him the rest of your team will hate you.
1st. For whatever reason Nassus seems viable now. Not "I can win my lane" viable but "I can survive this lane and farm" viable. So Nassus can actually do what he was designed to do: farm and farm and farm.
2nd. We all hate you. We all hate you so much. See, once the laning phase ends the rest of the team gets stuck in a perpetual 4v5 for every tower and objective. We will basically lose all of these fights but if we don't lose them too badly, you'll become op and we'll win. That's not fun for anybody else on your team because even if mid or bot won their lane, even dominated their lane, we're probably gonna lose a 4v5.
Don't tell me you have teleport or you're split pushing or something else. We all know you're not really split pushing. You're Nassus, you have Q, we know. We don't want you to teleport in, because at level 11 their Olaf is way stronger than you and we'd rather just have you keep farming Q so this will end quicker. The weakness of Nassus' kit is that he never really leaves lane and no quirky strategy is going to change that, it's integral to him.
So yeah, Nassus' early game is now strong enough to make him viable but his midgame is still weak. The only way around this is to have four strangers carry you through the midgame while you Q farm until you magically become op and we win. This strategy is completely viable, I've seen it work. We all just hate it because you've forced all of us to stall the enemy team for the majority of the game. We hate it and we hate you.