I used to have a fake Sleipnir and tried some other copies, I assure you that they are not even close quality wise, and the weight is different too.
I agree China have engineers and the machines just as good as the rest of the world, the thing is, it depends on the manufacturer itself if they want to push quality or not. A made in Japan yoyo made with low standards will be well, bad. When you compare YYR vs Rebellion, you probably have about the same standards because they are designed by Kengo anyway. And the price difference is mainly from the labor difference, not necessarily that they charged high on purpose because it’s made in Japan thus higher quality.
However, fake and real is not even comparable. My fake Sleipnir is not even playable anymore after a few dings because the body got bent.