Unfortunately what Google says is NOT always the best.
Sub-folders are preferred over sub-domains. Google says the two are equal, but data from tests and professional SEOs like Rand Fishkin, say sub-folders are far stronger.
SEO efforts like link building are dissipated from a sub-domain structure. If you launch into a new country, you don’t start from ground zero because of hreflang tags, but it is close to it.
Sub-folder usage would be most beneficial for stores with multiple domains that serve the same products to different regions. If WordPress could be setup in a subfolder, that would be a bonus.
Taken from my Shopify Expert SEO guide.