Yeah, Bruce Lee was only in, I believe, three fights since founding Jeet Kune Do. One of them was the famous fight to earn the right to teach his style to Westerners. But it actually was a poor showing. It took a really long time, and Bruce was winded afterwards. Eyewitnesses say he didn't really even win the fight. And the guy he was fighting was only halfway through his training (he only learned the external portion of his art at the time). Bruce was so discouraged with his performance that he basically scrapped jun fan kickboxing, leaving behind Wing Chun almost entirely and started again. The second fight was against Gene Lebell, a famous wrestler. It wasn't a REAL, REAL fight, but Bruce tried a few trapping techniques which Gene basically shrugged off and just grabbed him and put him in about a million different holds, giving him lots of nuggies in each hold. Bruce was embarassed, then asked him to train him in judo and jiu-jitsu. I can't remember the details of the third fight, but it was basically inconclusive and it's what caused Bruce to throw out his second incarnation of JKD entirely and start from scratch AGAIN.
The problem was (and Bruce himself admitted this) mainly that Bruce had a lot of speed, but didn't have what he called "stopping power", the ability to end the fight with sheer punching force. He fought against any number of guys who just walked through his hits and he found it to be a bit frustrating, so he began to integrate "stopping techniques" into his style, which mainly consisted of setting up a straight blast, which transformed into locking technique in the grappling range which could give him the option of breaking a limb. He was really trying to beef up his grappling game towards the end of his life, but he died before becoming competent in it. So JKD was largely incomplete. That's why a lot of JKD guys get owned at UFC championships.
That's also why I'd give Mas the edge. I don't think Bruce had enough power to really put him down short of dirty fighting. But don't think Mas doesn't have a bunch of dirty tricks up his sleeve.
One last thing - Mas wasn't undefeated. He lost two fights. One of them, ironically, was to a Wing Chun practitioner named William Cheung. The other was to a little known Brazilian dude named Gracie. 😉