I have personally explored all planets 100% - so Galactic Explorer title.
When I used ONLY a republic Character it was impossible to get the Makeb Exploration achievement. It was only after I'd taken an Imp character around that the achievement unlocked.
I think it's as others have said, you can't explore Braken Mesa last. I'm unclear though - those that are bugged, does it not work if you explore with another toon and make sure Braken Mesa isn't last?