Question About Jackshaft Kits.

Just out of curiousity before deciding to mess with one. Ive heard all the strong words against it but as someone who works on bikes full time and has even made some custom parts, Id like to see the flaws. Thing is it seems like theres no where to find instructions, sizes/dimensions, and any good knowledge overall. Ive seen the Sick Bike Parks version and PDF manual but didnt know if any of the dimensions from the parts theyre using crossover to the Chinese kits that are all that seem to be left going around. Any info is a big help as id like to see what I can bring to market as an improvement possibly.