The combo units are shit. Believe me,I tried.
Efficiency sucks, you can often dry less than you can wash. And especially if you have larger amounts to wash/dry you can only have half the throughput.
And according to a few customer protection/test organisations they seem to be less reliable as well.
Stay away from the combo units. They are shit. And the reduced throughput is a major issue.
A tower design has to have the washer below the dryer, as the washer is creating more vibrations and simply weighs more.
Not all manufacturers allow a dryer to be placed upon their washers and not all washers have a big enough top.
Some (Bosch-Siemens-Appliances does i.e.) do offer specialised “tops” that you can use and that are working quite well. (Example
Personally I would put another lashing strap around the combination, though,just to be extra safe, especially if you have children or pets.
Other than that the combination works without a problem,we have been using that for around 15 years by now, only interrupted by our experiments with combo machines.