Judging by the things Keyboardio ran into (they are also manufacturing in China), one of the biggest reason for sloppy manufacturing is the chinese "good enough" mentality: if it is okayish, not up to spec, perhaps, but okay-ish, it will be good enough. It is not a big deal when you look at all the little pieces, but when you require a number of components, and they are all "good enough", that will be a bigger problem. And most factories don't do everything themselves, but source components from various vendors, with varying qualities. So just sticking to a good manufacturer is not going to work, because they are still going to source some components from elsewhere.
Creating hardware is a kind of an art form, which pretty much requires you - or someone who can pay attention to details, and has the power to say no - to be on site quite a lot, until things get rolling, and all issues are ironed out, all questions answered. You can't trust assurances, because due to cultural differences, when you say perfect, you mean 100% on-spec. They may very well mean 75%, and honestly believe that you are talking about the same thing.
Mind you, this is just my impression I gathered from Keyboardio's adventures, and a number of other people who manufacture in China.