IIRC 9 speed MtB and road rear ders and shifters use the same pull ratio. Subbing a Shimano long cage MtB rear der (for the OEM road one with it's low gear limitations) is a well known way to get lower gearing on a road bike. However the front ders (and cranksets) use slightly different pull ratios and ring C-C dimensions. With a road shifter and a MtB der the front shifting will "sort of" index and work but there will be chain rub and/or less then better shifting. (And this is why Shimano at one time offered a flat bar STI shifter for road bike systems). (And this is why so many real touring bikes use bar end levers instead of STIs)

There's a bit of component tolerances drift and rider perceptions/skills that also are at play and when parts are new (or the rider isn't too aware/sensitive) mixed systems seem to work, sort of. Often after the miles of wear add up this "sort of" becomes less and less.

The brifters that have nearly zero compatibility issues with front set ups is the G springed versions of the Campy Ergo levers. But then you have the rear shifting issues of Campy's low gearing limits and their cassette fitting. Going to a Shiftmate adaptor can help intermix the various brands of drivetrain parts, although many will claim they are more trouble than they are worth. Andy