Quick take
For drinking water only, RO can reduce hardness minerals and many dissolved contaminants. For showers, laundry and scale throughout the home, use a real water softener instead.
Best drinking-water category: RO
RO can reduce calcium and magnesium at one tap while also addressing many dissolved contaminants. This is useful for kettles, coffee equipment and taste preferences.
Why a drinking-water filter does not soften the home
A countertop or under-sink filter treats only its outlet. It does not protect the water heater, shower glass or laundry from hardness scale.
Best whole-home category: ion-exchange softener
Ion exchange removes hardness minerals and is the correct technology when the goal is actual soft water throughout the home.
Do not confuse scale conditioning with softening
Salt-free conditioners may alter scale behavior but do not create the same chemistry as ion-exchange softening.