> According to SCS, the UI would be responsible for fetching that data from the other systems, and would pass all of it as part of the payload to the order fulfillment service.

One potential issue is that the order fulfillment service still needs to validate that the data its getting from the client is correct. For example, even if you send a list of full product details rather than a list of productIds to create the order, the fulfillment service still need a way to associate the Order entity with the Product entities. You could also send the ID with each of the products, but how would you know if the product has since been deleted or otherwise doesn't exist.

You end up needing the order service to either have its own set of knowledge about what products exist in the system for that user, or you need to make the synchronous call to ensure they're real products.