Interesting idea. I could see it being used in various places to capture low-level waste heat as steam only requires 212°F. The exhaust flue of a boiler comes to mind for example. Lots of energy being thrown away, but right now no practical way to capture it. You could put a few of these in the exhaust with a water source, and generate electricity. They would even work above a fireplace. Combined with mirrors to focus the sun they might well be more efficient than solar panels if used for that purpose. Drawbacks? The smaller ones seem terribly noisy but this is a problem all reciprocating engines have. A larger one operating at a few hundred RPM might be quiet enough for residential use.
One thing I noticed is these look like they could be made with cheap, low tolerance parts, not the usual tightly machined bores and cylinders usually needed for steam engines.