I've played several campaigns where the Carthagians became quite powerful and wiped out the Ptolemaioi, controlling all of Africa. I think they just need more time than other factions before they get agressive, but if they start expanding, they can get really nasty. In my campaigns they often start expanding in Spain and Sicily after 220. One time they controlled Italy, Spain, southern Gaul, the Alpes and started invading Germany around 200 BC.
When playing as the Romani I never had a problem with them in Sicily, but the longer you wait, the harder they fight in Spain and Africa.
On the other hand its sometimes annoying that they are never a threat for the Romans when I play as Aedui or Sweboz, they don't seem to break their alliance if the human player is at war with Rome.
I've only seen one single naval invasion in EB with RTW 1.5, but unfortunately not from the Carthagians, no idea if they do better with BI.

Btw, as this is my first post here, I have to say: EB is totally awesome!!!