They’re the most reliable source we’re going to get without being party insiders. There’s still Soviet-style inflation of figures to meet quotas but China has been cracking down on that for the last few decades because they want accurate data for the five year plans. I think it’s more of a problem with outer provinces, less so for the major manufacturing hubs.

Alternative sources to verify are a bit harder to find without knowing the languages (lots of the NRDC and NBS stats are available in English).