I'm looking at replacing my old alum windows and panel siding with new construction vinyl windows and fiber cement lap siding. It seems like integrated brick mold/J-channel receptors are becoming very popular because of the time/cost savings compared to installation of traditional exterior trim with a drip cap. They say once the header nail fin is sealed with flashing tape, you don't need metal flashing above the header brick mold because the nail fin has been incorporated into the building envelope.
But to my way of thinking, it doesn't make any sense to channel siding run-off above the window into the header J-channel and down the jamb J-channel behind the terminated siding. Sure the window perimeter is sealed with flashing tape, but the design virtually guarantees water behind the siding below the sill. You could caulk the joint between the header brick mold and lap siding, but it will eventually fail, you could place a felt paper spline under the sill to redirect the water out to the siding exterior, but why would you want to allow water to be routed behind the siding in the first place?
Am I missing something?