From the www.W5SWL.com FAQ of 2 July 2018
They are good, it used to be one of the best because in our opinion there were few connector manufacturers. Now a lot of connectors are now made on a China Line. Being made in China is not the end of the world, as a manufacturer you just have to watch over the production and have a vested interest in the line and quality of the product. Would I buy a name brand connector made in China, sure I would as I know they exercise oversight of the product.
Some lines in China perform well in manufacturing, many however don't and only look like a RF connector. Believe me when I say I have tested 100s of China connectors and only a small percentage of them pass my tests. In other words, they might be better used passing water than RF.
Again, not to be said of all China connectors, but if you buy 10 BNC connectors from China for a buck, well you will get what you paid for.
Taiwan and some other lines that make connectors are mostly pretty good.
The real secret is to know who you are buying from, does the selling person understand RF and know how to check things or is it a eBay seller with the user name similar to Your-YoYo selling everything from panty hose to connectors. You get the idea.