SA strain is fundamentally different from most other strains in the world atm. It's a resistant mutation, in essence; the core fundamentals of the vaccine function, which is why they expect to be able to push an update by around August. The timetable would speed up if this strain started to affect a country with the cash to actually afford better research, mind.
The main advantage of the J&J variant is that it doesn't require cold storage and is thus easier to transport and distribute, especially to places with poor infrastructure. There's significantly less chance of New-York style "just throw it out" as well, since the stuff doesn't really spoil quite as easily. It also doesn't require a booster, iirc.