Do most stackable washer dryers fail because of the washer being overloaded/won't drain?
Rather than simply "stacking" the dryer on the washer (as the term implies), it's important to secure the dryer above the washer with a solid connector. The stacking kits provided by manufacturers accomplish this task. The problem is not just large vibrations, but frequent tiny ones that cause them to move away from each other - similar to a vibrating phone moving on a table.

The dryer is commonly (if not exclusively) put on top of the washer for weight reasons, as you suggested. The washer will be filled with water, whereas the load in the dryer will weigh much less.