Three substrates turn up in bathroom cabinetry and they behave completely differently when wet. Melamine-faced particle board swells and does not come back: the fibres burst, the face lifts, and the damage is permanent from the first soaking. Moisture-resistant MDF shrugs off humidity and short spills and is still not waterproof. Exterior or marine-grade plywood takes a genuine wetting and dries out more or less intact. What separates them is what happens the one time a supply connection weeps behind the plinth for a fortnight.
Failure starts at an unsealed edge near the floor. Every carcass has cut edges nobody inspects — the bottom of each side panel, the underside of the base, the back face of the toe recess — and those are precisely the surfaces water reaches first and sits against longest. They are also the ones a production finishing schedule skips, because they cannot be seen. Banding all the way round rather than only on show faces, or laying a coat of finish on the underside before the box is assembled, adds minutes at the bench and decides whether a small leak is a mop-up or a replacement.
Finish on the fronts follows the same principle of covering what is hidden. The machined edge of a painted panel is porous end grain that drinks coating, so an unsealed profile telegraphs its fibre through the topcoat within a season. The top edge of a drawer front is the horizontal surface water actually stands on, and is routinely the thinnest film on the whole door set.
Hardware fails at its fasteners rather than its mechanism. Steel screws and the mounting plate spot and then seize long before a hinge arm gives up, so the corrosion rating of the whole assembly matters more than the make of hinge. On drawers, undermount runners hang the load off the drawer bottom, which means that bottom has to be a proper panel captured in a groove rather than a thin sheet pinned underneath. A drawer of bottles is heavier than it looks, and what carries it is the runner rating and the joint at the front corners.