There are only 4 systems at the moment reaching South Africa. EASSy and SEACOM both run around the east coast of Africa, with WACS around the west coast. WACS is not yet live in SA (and therefore we don't know if it goes down), and SAT3 has connections around both sides of Africa, so if either side goes down the cable system as a whole isn't down. Neither SEACOM nor EASSy are redundant, so failures immediately affect their users.
Then there is political issues, the pirates in Somalia, the revolts in places like Egypt, SEACOM's troubles with SEA-ME-WE-4, etc. We will have to compare WACS to the two other systems when it comes online to see what the difference in reliability is.