The problem with those units is that you can't start a new load in the washer before the previous load finishes in the dryer, because there's only one drum. So if you want to run several loads, it takes twice as long.

But you could have two of them in the same area.

If you've got space for 2 machines, you can increase energy efficiency by using the specialized machines.

You waste a lot of energy heating up and drying the drum, too, compared with a conventional dryer, where you're only drying the actual clothes.

I have one and it's awesome, Europe has smaller homes do they are much more common here I think.

I had one once, an all-in-one combo unit with the ventless dryer. The mechanism it used to dry clothes didn't work very well, and the clothes never actually got fully dry.