Any bong sao, done incorrectly, can result in injury and be non-functional. Conversely, any bong sao (regardless of height or passivity) done correctly, can be injury free and functional.
When I learned, we had several heights for Bong Sao, including one high Bong Sao (Pao Bong or throwing wing), and Bong Sao that were both passive (responded to pressure) and active (bridge seeking or structure attacking).
Historically, it was said the high Bong was particularly useful for defending against certain types of South Fist attacks (see: Choy Lai Fut), in modern/western contexts, IMHO there are still angles and types of attack that high Bong Sao offers optimal defense from. (Big Note: you must have, of course, developed the flexibility and relaxation necessary to perform high Bong without breaking your own structure or becoming stiff - very common problems!) An example of this Bong Sao, and its training, are in Siu Lien Tao.
In personal experience I’ve met lots of folks from lots of lineages who did not agree with high Bong Sao and to the best of my recollection, all of them were vulnerable to certain types of head attacks. Perhaps 90% of the time these attacks aren’t encountered, but IMHO, WCK has tools to cover even the other 10%.
As to active Bong Sao, this can cross into the covering thread (sweeping bong sao can cover an area, vertical or horizontal and we have examples of it in Chum Kiu). Mid-level throwing wings can also be used to attack joints or smaller bones (ribs), once the Ging is developed (Bong Sao develops some nice Ging IMHO
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WCK, again IMHO, is yin & yang, and favors the center. When something is done a certain way, we can often look and find its match the other way 8)
Rgds,
RR