This was ~10 years ago, I'm sure the current state of the ecosystem is very different (no idea), but this is the reason why I was never able to adopt it, and I heard the same thing from dozens of Flash creators at the time.
At some point I should really do more with it and make a game or something.
It came up a lot when making HTML-based forms. I’d define them all in PHP, but the experience is best without a page load, so typically I define some typescript object to send the form via Ajax. I got tired of having to define all the field names in both places (awful for maintenance), so instead typescript uses AJAX to ask the server for the list of inputs it needs to grab and send.
It sounds like Haxe could be a much nicer way to do stuff like that: https://haxe.org/use-cases/web/
Haxe might be overkill for that. Usually (unless youre doing a SPA) you use PHP or a templating language to dynamicly render the form server side based on either data or metadata.
Haxe can indeed be useful for more complicated versions of that situation, where you need a complicated state machine or algorithm in more than one language.