Complex sentence: With the introduction of the Atlas V EELV, balloon tanks and "1.5 staging" are no longer used on the Atlas rocket--only the Centaur retains this feature and has not undergone a single catastropic failure since the Atlas V's first flight.
Split into simpler sentences: With the introduction of the Atlas V EELV, balloon tanks and "1.5 staging" are no longer used on the Atlas rocket. Centaur, however, retains this feature and it has not undergone a single catastrophic failure since the Atlas V's first flight.