Mechanical shifting relies on housing maintaining its length so cable movement positions the derailleur accurately. Ordinary longitudinal-wire shift housing is unsuitable for brakes: braking loads can cause it to burst.
Traditional brake housing uses coiled steel and compresses slightly under load. Low-compression housing intended for braking is specifically engineered for that task. The word “compressionless” alone therefore does not establish suitability; check the manufacturer’s stated application.
Stiff or inconsistent shifting can come from kinked, worn or corroded cables and housing. Routing should use smooth curves and leave the handlebars free to turn. Ferrules must suit the components and seat fully; choose parts against the system’s specification.