The Customs data is well recorded by customs.
As everything goes through the customs, so the customs data is the best source of data. If a country’s customs data is available, then there is no need to look at shipping data anymore. But not all countries’ customs willing to disclose. Countries like Japan, Singapore, Malaysia, Thailand don’t disclose at all.
Shipping data is gathered from shipping companies. Not all shipping companies willing to disclose the data. so, the completeness is not as good as customs data.