HATEOAS, for anyone who’s not come across it yet, is an additional constraint added on top of normal REST services whereby the only way that clients ever interact with the server is through Hypermedia that the server provides. The server will provide a single starting point, and every single interaction with the server after that is made only through URLs that the server has previously provided to the client. The advantage here is that the server can change independently of the client, and as long as the client only ever follows the URLs provided to it then everything will continue to work correctly.
That all sounds fantastic on the face of it. Completely decoupled evolution of server and client sounds almost idyllic.
Of course, in reality this doesn’t actually work. There are a number of problems with the idea, ranging from the very simple - clients cut corners and just hard-code URLs to specific resources instead of following links - to the very complicated, where changes to the server need to be made that just aren’t backwards compatible.