I never trained with Bruce, nor met anyone who ever did, but I have been involved in the Chinese martial arts world since the late 1970's, and most people with any idea say Bruce came to America with a portion of the wing chun system, couldn't get it to work against bigger Americans (and an old Chinese dude he fought a challenge match against) and decided to modify it with other stuff - namely boxing - and thus started his own style (named after himself - Jun Fan gongfu) - which later became JKD as he became a little more philosophical about the differences or lack thereof between styles.
By all accounts, he was someone with incredible control over his body, but largely untested as a fighter.