First of all I want to say that I have no side on any issues about Shaolin do. I have my own opinions about SD, but my research is a firm attempt to remain objective and seek the truth.
The dates of the SD grandmasters fit into historical accounts of events during the times that they lived. This does not mean that they existed, just that their existence was possible.
Guang Xu (Manchu) was emperor of China from about 1890 until the Boxer Rebellion. Some historians claim that the Fukien temple was destroyed completely just before the Boxer Rebellion. This would place Su Kong (the hairy one) at the right time and the right age to have been at the temple and to have escaped into the mountains. The best report I can find is that about 30 or so inhabitants of the temple excaped, most are unnamed.
The next grandmaster who killed 11 soldiers also fits into history. Around 1928 when he was supposed to have been awarded this title, China was not under a unified rule. The camp of a warlord that he wandered into could have existed. Also, it is reported by historians that many practitioners of kung fu traveled south to maintain their safety. The move to Indonesia fits into this.
Sin The is a real person. He attended the University of KY with the father of a friend of mine. He is Chinese (my friend's father is Chinese). My friend's father knew Sin and I asked him some questions about him. He said that Sin was a good engineering student and lived on the same floor of the dorm with him.
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So much for objective, now for my opinion.
I think SD looks so much like Karate because they have the same roots. And the roots aren't that deep. Karate practitioners who make up the lineage of the four main Okinawan styles (Shotokan, Shinto-Ryu, Wado-Ryu, and Gojo-Ryu) traveled to the area of China where the Fukien Temple was located. Regardless of the grandmaster's lineage being legit, SD most likely came from southeastern China as did the roots of Karate. Tode Sakagawa who by most accounts is the first major practitioner of Karate in Okinawa, lived around the time that the Fukien Temple was supposed to have been destroyed. It is possible that he learned martial arts from the same people as Sin's martial ancestors learned from. Of course the martial arts of both lines evolved independently due to the envoronment, politics, and the individual practitioner's physical capabilities and motivations. But they are not seperated by much time, so the styles still share much similiar material.
From what I have seen of SD I believe it to be an effective combat martial art. Just as effective as Shotokan, Jujutsu, or Wing Chun. Of course it has its weaknesses, but all styles lack somewhere if we are persistent enough to look for them. If someone is training to be combat effective I think SD is a good system to utilize in a cross-training program. I have found through my own experience that the instructors are willing to help in this cross-training endeavor. When I went to the Shaolin-Ryu club at UK while I was in college, they let me keep my rank from my own style and allowed me to train with students at all levels. I had a good time with these guys. Their class sessions were physical and I got some good exercise from both kata and sparring. I learned some things fighting with them, and I like to think I taught them something too.