"Modified in the USA"? What makes you say that? While it is certainly possible, I have serious doubts that this is the case. Based on the two I've installed, these units are factory built and packaged, shipped in containers from Shanghai, arrive ready to sell and deliver. They are turn-key, everything packaged together in sealed boxes with substantial packaging. Frankly, the only other international products I've seen packaged like this is the stuff that comes from IKEA.
The only reason I can think of that there would be returns is if there is a serious issue with component failure - which I doubt since millions of these type of units are made and sold all over the world - or from user error on installation. It's not as easy as they make it out to be, and every installation varies a bit due to site conditions, walls, rooms, terrain, etc. And you need a direct-wire box with 115v for the 12K unit, 220/240v for the others. For most people that presents a challenge without hiring an electrician.
Yes, will be interesting to see. I think the only reason more of them aren't sold is that people are just not very DIY savvy these days. Most can't even pickup a hammer or screwdriver, don't change their oil, can't even build a simple wooden project. [shrug]
A recent visit to Home Depot in the afternoon revealed the majority of the customers looked like handyman laborers. I'm sure this chain's success depends on DIY'ers, but I don't think it represents the majority of homeowners. I could well be wrong however.