We're considering getting a stacked washer-drier (top-loading washer). I like the option of soaking clothes before washing them, which is why I like a top-loader.
What are the drawbacks / things to consider when buying such a unit?
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).
My primary concerns with stacked (single non detachable unit) appliances is durability. In my experience dryers outlast washers at least 2 to 1. I have had three washers in my life and still use the same dryer. Dryers, at least the low tech variety only have a few items that fail. Belts, timers, bushings, etc. tend to be inexpensive and easy to replace. If the washer tub rusts, the gearbox or pump fails and repair costs exceed the value you have to throw away a perfectly good dryer.
One obvious drawback is that top-loaders use more energy, water, and detergent than front-loaders. You can use a pre-wash or rinse cycle to give clothes more soaking time in a front-loader. See also: How to Solve the Soaking Problem with a Front Load Washer
There are stacking kits for separate washers and dryers of the same brand, including front-loaders.