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It might be worth mentioning to the parent poster that SSEs (and Mercure) are unidirectional only (server -> client), whereas WebSockets are bidirectional.

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Nowadays that's hardly an issue because SSE is multiplexed over HTTP/2 and HTTP/3 alongside regular requests. In some ways, SSE is today more efficient than websockets too because if you use it with HTTP/3 then you're avoiding head of line blocking which is still an issue with websockets.

(Also in practice many places websockets are not even supporting HTTP/2 because on the server they were implemented on top of raw sockets which is no longer possible with HTTP/2)

Head-of-line still exists in HTTP/3, but on another level. A stream can still get blocked. With websockets the whole connection gets blocked (HTTP/1), but that connection isn't shared with anything else, right?

Worth noting that HTTP/3 seems to recover faster than TCP-based protocols.

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