Just curious, why does the service manual for every level of dual control levers contain this warning?

"Because the high cable resistance of a frame with internal
cable routing would impair the SIS function, this type of frame
should not be used."

As about 90% of the new internally routed frames I see have either Shimano 105, Ultegra, or Dura Ace levers on them, there is obviously not a real issue. I'm guessing there must be an interesting story in the background somewhere.

Cable routing is largely left up to manufactures other than Shimano - frame manufacturers, handlebar manufacturers, etc. This is one of the biggest points of variability effecting the products Shimano makes. I have seen some very poorly designed internally routed frames (especially from some of the pop-up mail order brands).

If you're Shimano, you probably don't want people calling your product bad, when in reality it could be the customer's frame not having the greatest routing.

+1

Try setting up one of these frames for brakes and derailers, preferably one of the really wacky Tri frames. Then you'll see why Shimano makes this claim. Shimano is indeed covering their butts, but real problems do exist with some of the frames.