日本購買產品在臺灣無法保固
日本購買！ 臺灣無法正常提供保固，因為內部相同產品，但內部結構不同，因此等於無法正常提供保固服務
That is correct, the built in hardware has to support the local carriers, as mentioned in this related article:
Apple Watch - Carriers - Apple
And the warranty can also be restricted by Apple to the country or purchase.
Legal - Hardware Warranties - Apple
Plus, the watchOS will also be country specific for all watchOS features, which can not be changed as well.