In the year since 4.8, confusion about how to properly expose meta information through the REST API has been one of the most prevalend issues we've seen in support posts and tickets.
Most developers try to use register_meta with show_in_rest => true; unfortunately register_meta works on object types, not subtypes, so you cannot pass a custom post type name into register_meta like you can with register_rest_field. Worse, if you register meta for the type 'post', then that meta key is registered not just for core posts, but for every single custom post type. Meta almost never applies across all post types, so this behavior forces developers to use register_rest_field for REST API data modeling instead of taking advantage of the API's built-in meta handling.
there is no way to and this behavior makes it impossible for a developer to register meta that is specific to their single post type. and requires any REST API data modeling to use register_rest_field instead of register_meta.
Despite discussion about adding a subtype argument this got backed out of