I haven't bought one and wouldn't. Why not save a grand? The reason lies if you have ever known anybody who has had a life changing crash. I have.
So why is a knock off frame a risk? Because even the best brands in the world have a risk of failure. And I know the discipline of testing that occurs with the best brands to avert liability and those disciplines that aren't adhered to with no name brands.

So what is the risk of failure even with all the tremendous R&D of top brands? It happens in spite of all the due diligence. Guess what happens without this discipline? A lot more failures.

Every top brand has had recalls due to frame failures. When do top makers implement a recall? Generally when people have gotten hurt. This is when large companies find out about discrepant frames and weigh the cost of the recall which can be massive versus the potentially greater cost of liability and risk to sink the brand.

In the case of Specialized, they had fork failures on their top race bikes. Procedure was to remove the fork at a Specialized bike shop and ship the forks back to Specialized for X raying and risk assessment. Not all forks were bad and its hard to determine what forks are good versus bad. Recalls can also occur and in fact are common even when there is an interaction with customer poor practices. In other words, using the proprietary expansion plug with a Specialized bike incorrectly by placing spacers above the stem which causes the steerer to be unsupported at the base of the stem where stress is the highest...Specialized has to design for this common contingency which they do.

How much due diligence do knock off brands perform? Little or none. Your body and the choices we make. Not all products are safety critical. Bicycle framesets are.