Complex sentence: Most such US projects to create such vehicles have been abortive, e.g. the M8 Armored Gun System, the most successful, being the flawed M551 Sheridan light tank.
Split into simpler sentences: Most such US projects to create such vehicles have been abortive, e.g. the M8 Armored Gun System. The most successful was the flawed M551 Sheridan light tank.