I have seen too many Kickstarter projects crash and burn because they read guides like this, speed dated some factories, and then lost all their money in a bad production run, by a factory who doesn't really care about them.
Unless you work with a factory who you not only trust, but know that they have an extremely strong motivation to not steal your product, they will both screw up your product, and your design will be cloned in a matter of weeks.
Even friends I have who are Chinese, living in Hong Kong, have warned me that even for them, it is hard to find trustworthy suppliers and factories in Shenzhen.
We are going to be doing all our manufacturing and assembly here in Europe. Luckily we're making a more expensive item and we have this choice.
" But if you’re undertaking a serious endeavor and you’re looking to make mission-critical parts, you have to visit to really know the situation at your suppliers, to establish relationships with factory owners and staff, and to get a sense of their quality control standards. " NO NO NO NO NO. Unless you are going to MOVE THERE AND LIVE THERE, then you need to absolutely forget about manufacturing there. You WILL lose.
I don't want to sound down on Shenzhen - it is where almost all our beloved electronics come from - but seriously - it is an environmental catastrophe, there is ZERO intellectual property protection, it is full of scam artists, there is a nearly insurmountable language barrier, and yes, there are in fact plenty of completely competent, worthwhile, trustwothy electronics manufacturing assembly companies in nearly every country of the world. Unless you are going truly mass-market (>200,000 pcs) then you should absolutely not even consider taking it to Shenzhen.