Agree on washer on bottom - due to the weight of a washer vs dryer. Not to mention the additional water weight and agitation of wet clothes (in water) thru cycles when in use. Our stick built has a stackable washer/dryer and it is a good space saver in our utility room.
The new all-in-one combo units definitely make sense for RV units and small stick built applications. Although much more efficient now with larger capacity, the concept has been around for awhile. My parents had a relatively small "all-in-one" combo while living in an apartment in Germany in the 70's. The biggest difference in 50 years is capacity and a "some" better drying. But just like the ones in the 70's, they still have some limitations. They are not very efficient at drying clothes completely. The newer ones still require some "compensating" to get clothes completely dry by finishing with hang dry, multiple drying cycles, or smaller loads. Our son has a newer front load washer dryer all-in-one combo. He mentioned its not as efficient at drying large volumes of clothes, and has to wash smaller loads than capacity to get dry on the first cycle.