Carbon quality can make a huge difference, and I’ve had first-hand experience with a track frame. Riding an adequate but relatively inexpensive frame from one manufacturer, hard sprint efforts would cause noticeable flex. However, as the frame was an ‘open mould’, ostensibly the same frame from another manufacturer, albeit costing more, was noticeably stiffer due to the lay-up and quality of carbon used.
From any ‘known’ manufacturer, although there might be differences in terms of stiffness, weight and performance, safety shouldn’t be an issue. But there’s no doubt that ‘too good to be true’ Chinese fakes shouldn’t be trusted.
Although the frame, wheels or components may look identical to the real thing, inevitably carbon quality and, potentially, structural integrity will be inferior. You might get lucky but, if you’re hurtling down a descent, is it really worth the risk?
Unfortunately, there’s no way of telling the quality of carbon externally, the only answer to ensuring quality is to stick with reputable brands.