Quality control is always a concern when working with an overseas vendor, and I can only imagine your frustration with the samples you have received. My advice having sourced from China and many other countries is that you need a person on the ground to make sure your expectations and specs are being met, so see if you can find an agent (not associated with the factory) to work on your behalf to save the time, cost and hassle of having to fly overseas. That said, unless you are at a scale where it makes sense, I have found over the years that it ends up usually being cheaper in the long run and much more efficient to produce domestically if you can. While overseas pricing can seem a lot cheaper on the front-end, if you run into issue (like you have), the savings end up evaporating. Hope this helps, and happy to jump on a call to talk in more detail.

Sample products not working is not a good sign, and typically unusual. Normally the samples always work perfectly. It's usually the subsequent shipments that have issues in quality. I don't mean to say this applies to every Chinese company, but it does happen quite a lot. Obviously companies in China are becoming more and more professional, and it's no longer difficult to find companies or factories that do very professional work. However, there are many companies there who look for a quick buck, and since they have an unlimited source of potential customers globally, they don't necessary care if they lose their customer, after an initial order. That's essentially how some companies in China earn money on platforms like Alibaba and Taobao. They showcase a product that looks and sounds amazing, until you get the actual product (typically in higher quantities) and find it to be different than advertised, or of a much lower quality, or lesser features, or not functioning at all. They are not expecting return customers in most cases. Most of their money is earned by first-time buyers who haven't researched the company or the product well enough. I know this sounds like a scam, but truth be told, China does have many such suppliers and it's not easy to spot them if you are working with China-based vendors for the first time. Also, keep in mind, that as a foreign buyer (not based in China), there is very little you can do (legally), in case you have been tricked or cheated by a Chinese supplier. My recommendation would be to first do your research thoroughly on the company and their product. If you are unable to do that, and also unable to find a local representative/agency to verify this company's history and quality, then the safest option would be to work with this supplier on credit/payment term basis, where they ship the product first, you verify all the units are functional, and then pay them within 30,60 or 90 days. Another safe option is via Bank LC (though beware that many Chinese suppliers refuse to accept this and prefer advance TT only).

PS : Samples usually always work. They are either very professional or very unprofessional if half of their first samples were not be working. I would advise caution.