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One limitation of SSE compared to long polling (and WebSockets etc) is you can't efficiently send binary data such as cbor, protobuf, etc. Though if your long polling is chatty enough eventually the HTTP overhead will kill your efficiency too.

You can use binary-sse (https://github.com/luciopaiva/binary-sse) with minimal overhead.

Long polling is more amenable than SSE for most HTTP tools out of the box, eg curl. The SSE message body is notably different from plain HTTP responses.

To the OP, you can still build APIs with long polling. They are uncommon because push patterns are difficult to design well, regardless of protocol (whether long-polling, SSE, websockets, etc).

Whiteboarding a push API is a good exercise. There is a lot of nuance that gets overlooked in discussions whenever these patterns come up.

Well I know I can write applications use it, but I don't often write code outside the context of a team that has other opinions anymore :)

Agreed - I see SSE as basically a standardized approach to modern long polling