Is a washer-dryer a good idea?
It saves space and compromises on almost everything else — and the specific compromises are predictable enough that the decision is straightforward once you know them.
The genuine advantage: one appliance in one space, with one set of plumbing and one electrical connection. In a flat or a small kitchen where there is only room for one machine, that is decisive and the rest of the analysis is academic.
The compromises:
Drying capacity is substantially less than washing capacity — frequently around half. A full wash load must be removed and split to dry, which defeats much of the convenience and is the single most common complaint. Check both figures, since the headline capacity is the washing one.
Drying is slow, frequently several hours for a modest load.
Traditional condenser washer-dryers use water to condense the moisture from drying — sometimes a considerable quantity — which is both wasteful and expensive where water is metered. Heat pump washer-dryers avoid this and are far more efficient, and considerably dearer.
Energy consumption is high on air-condensing models.
You cannot wash and dry simultaneously, so throughput is much lower than two machines. For a household doing several loads, this is the practical limitation.
More complexity in one appliance means more to fail, and a fault takes out both functions at once.
Repairs are frequently more expensive, and the machine is denser and harder to work on.
Wear. The drum and bearings carry both duties, and drying introduces heat cycling.
Who they suit: single people and couples in small properties; households with genuinely no space for two; and anyone using the dryer occasionally rather than routinely.
Who they do not: families washing daily, anyone drying most loads, and households with space for two machines — where a separate washing machine and a heat pump tumble dryer will outperform on running cost, capacity and reliability.
The alternative worth considering where space is tight: a washing machine plus a heated airer or dehumidifier, which dries effectively at low running cost and takes no permanent space.