they work, they are small (DW size) and they are slow. You can only fill them halfway due to it being a washer and a dryer drum. It is really a living space size consideration. if you have the space you can get two compact units (washer and dryer) - better choice

You can stack a compact w/d set. The width is 24" and the depth is something like 23" but you need to accommodate for utility hookups. These units do fit under the counters.

The combo units do work, and they will fit under a kitchen counter, but you will need to allow some space for ventilation. They are slow, though.