Short answer
Colour drenching — taking one colour across walls, ceiling, trim and sometimes joinery — works best in small rooms, rooms with awkward architecture, and rooms you pass through rather than sit in. It works least well in rooms with fine period detail you want to keep visible, and in rooms with a lot of fixed white you cannot change.
Contrast is what makes the eye register a boundary. Painting walls, ceiling and trim the same colour removes those boundaries, so the room stops reading as a set of surfaces with edges and starts reading as a single volume — which makes small and awkwardly shaped rooms feel larger and more resolved rather than more enclosed. The corollary is that the technique erases detail: a moulding picked out in a contrasting white is emphasised, and the same moulding drenched disappears into the wall.
Cornice, panelling and architrave are the things drenching hides. If they are the best thing about the room, drenching is working against you.
Radiators, uPVC frames, sanitaryware and a white-framed window will interrupt a drenched scheme, and interrupting it is exactly what undoes the effect.
Drenching a dark colour in a room with almost no natural light is a real choice with a real result, but it will be a dark room. Drenching a mid-tone gives the same seamlessness with far more light.
Using matt on the walls and eggshell on the woodwork in the same colour keeps the boundaries invisible while giving the trim the durability it needs — this is the standard way the technique is executed.
Use drenching carefully in rooms used for detailed tasks — a home office or kitchen drenched in a deep colour will need considerably more artificial light than a conventionally decorated one. It also makes future changes harder, since every surface has to be repainted rather than just the walls.
How to test it
Test the colour on a wall and on a board held up flat against the ceiling. Ceilings receive very different light from walls, and the same colour overhead almost always reads deeper than it does at eye level.
Undertone
Drenching amplifies undertone more than any other approach, because every surface reinforces every other. A colour with a faint cast on one wall becomes a room with a definite cast when it is on all six surfaces.
Lighting
A drenched room has no white surface left to bounce light around, so it will be measurably darker than the same room with a white ceiling. In a room already short of light, this is the deciding factor.
Finish
Ceilings in a mid-sheen finish show every imperfection under raking light from a window. Matt on the ceiling is the usual choice even when the walls are eggshell.
Whatever a calculation or a chart says, view a physical sample in the actual location before committing to a large surface.
Colourwise editorial guidance, drawn from conventional decorating practice and from the physical behaviour of light and surfaces described in the linked lighting and finish guides. Not manufacturer advice — always follow the specific instructions supplied with any product you use.