I'm the GP, not the parent, but I'll give you some of my complaints (other than the already mentioned infuriating timeout).
* The Metafields feel both over-engineered and difficult to use. Stripe did a great job - metadata is a set of string key/value pairs that are always fetched when you fetch a thing. Shopify's metafields are typed and need to be fetched explicitly from the REST API. You can't get a list of Products and their associated metafields; you have to fetch each Product's metafields one at a time. It's a critical facility and yet works very poorly.
* The API for metafields is inconsistent. Want to get metafields for a variant? /admin/products/#{id}/variants/#{id}/metafields.json Want to get metafields for a product image? /admin/metafields.json?metafield[owner_id]=#{id}&metafield[owner_resource]=product_image WAT?
* Variants (and images) have unique ids, but you always have to reference them as /products/123/variants/456. Variants should be /variants/456.
* Suffixing all the requests with .json is annoying and not RESTy. We have content negotiation headers for that.
* Yes, I know you can work around some of this with GraphQL. Does Shopify intend to deprecate the REST API? I am not personally a big fan of GraphQL, but if the REST API is getting dustbinned, I'd like to know about it.
Mind you, this is just what I noticed from a few days of working with the API.
PS IMO, 3x isn't much of a flash! 300x, now we're talking :)
