Mundo typically excels in the late game because of how well he scales with items. His early game is pretty weak, but between his durability and his ultimate's health restoration, he can survive a lot of punishment and his damage is still significant enough to let him 1v1 most champions without much issue.
End game, my typical Mundo build looks something like this:

It lets you duel pretty much every champion in the game, either directly or through kiting with cleavers, and gives you enough durability to survive in teamfights. He's a pretty good splitpusher because of his high waveclear (Sunfire Cape and W), how difficult he is to catch (Ultimate MS, Cleaver slow), his ability to push towers (E AD steroid), and how difficult he is to actually fight late in the game. He's also a huge nuisance in teamfights because he deals a lot of consistent damage, if very difficult to peel off squishy champions, and is extremely hard to kill without focusing him (and things don't go well when you focus a tank in teamfights).
Considering how his sustain, ability to farm with cleavers safely, and teleport can let him easily survive his weak early game, so I'd definitely rate him as 8,9, or 10 late in the game.
However, keep in mind that Morellonomicon is still a popular purchase for AP champions, so they will have a source of Grievous Wounds aside from ignite.