You'd get the order(s) embedded with their invoice in GraphQL in a single request, applying the criteria to the invoices. Or you'd get orders matching the criteria with its invoice(s) inline. Doing two requests would be way more complicated for the developer to write.

Nonsense. As someone who's spent the last year and a half working with Shopify's API, it's far more complex to formulate a single request and then extract all the parts out of it. Especially if there's any kind of pagination involved.

Shopify's REST API is way easier to use, it's just slower. If pipelining solved the latency issue, it would be an all round superior experience.