Drawbacks of a stacked top-loading washer + drier
Stacking a top loading washer on top of a front loading dryer is going to be a ripping pain to use. The height to the top of the stack will, in the worst case, require a step ladder to get the cloths in and out. If it is reduced height thing then maybe just a stool is required but it still seems like a big pain. 

There is another consideration too. Washers, especially ones loaded with wet cloths and water, are way heavier than dryers. It seems all wrong to put a heavy washer on the top of the stack.

Lastly if you select one of the type of stacked units that only lets a lower washer door open up to a 45 degree angle I find those to also be a pain to reach into and get the cloths in and out (especially out).