That's correct, you destroy HTTP-level caching.

(For one, your queries can be large so are POSTed anyway.)

Relay is a caching layer that understands GraphQL semantics, but you no longer have transparent HTTP proxies, browser caching, etc. Though HTTP proxies are dead thanks to HTTPS, and with modern browser APIs, you can do your own limited caching.

It's certainly a tradeoff for what you expect to give you best performance gains: fewer roundtrips, or more caching by third parties.