On top of the already-mentioned JS interop breakage, Elm 0.19 also dropped native Websocket support[0]. The API had issues (fair), so it was dropped rather than improving it due to wanting to do it perfectly (okay, I guess), buuut due to the JS interop restrictions this meant that 3rd-party experiments or alternatives were impossible (??), which meant that any use of Websockets was in practice now completely impossible! If I recall correctly something similar happened to other core libraries.

This "nobody is allowed to do this until Evan himself has made time to come up with a blessed solution" style of development left a lot of people quite disappointed. Elm was marketed quite heavily as the best thing since sliced bread and the future of front-end web development, but in reality it turned out to be just Evan's toy language which you could look at but weren't allowed to touch. Which is of course allowed, but it does rapidly kill any kind of community around it.

[0]: https://github.com/elm-lang/websocket