I used to work with Actionscript and Haxe came from the guy who wrote Mtasc, so we were quite excited. We loved that you could access features of the Flash VM that were not available through Actionscript, but it never gained traction. Right now, Haxe is a weapon of choice for people building Smart TV apps and things. I think that Nicolas was working for Massive Interactive doing just this.
I can only answer this as someone who's looked into Haxe for a couple of years now versus someone who has actually made an application/game/website with it, but from what I can tell, it's really the 'good at most things, great at non' option for easy interactive, multi-platform development.
What you may not know, is that HaXe started off as an alternative to using Actionscript to build flash/flash-like projects, without having to use Adobe products directly. When the bell started ringing for everyone to get out of the Flash pool (helped a lot by Apple not including it in IOS), they sort of pivoted to being the open source version to Flash (OpenFL), which they used HaXe to build. (One of the things going in their favor was that they were one of the 1st tech stack that tried to do what flash did in a compiled language that included mobile and web targets, besides desktop. The only other ones I can think of that tried was Java and a bunch of 'compiled' js and html files into executables.)
Once they realized that they could just change the target of what they were cross-compiling to, they started adding other targets as well (started with swf and actionscript, then went Javascript, Java, PHP, C, Ruby and who knows what else). People started adding other things to it, such as IDE, game engines, and even Adobe themselves are re-purposing Adobe Animator to target OpenFL. And it grew from there.
Where I usually get confused and leave it all alone is that there is so much churn in that area, that it's hard for me to even begin pick a stack to work with. Apache Flex competes in a lot of what HaXe does, same as Apache FlexJS, Apache Cordova, Cocos2d, Dart/Flutter, Monogame, and so much more in the same area. It got to the point where even Unity and Unreal engine provides much of the same features, pretty much for free until you've hit a certain monetary limit (I think both are around 250k to 1mil, USD).
So, to answer your question again after I've bored you, the catch is that it's one of many (and I do mean many) options for you to choose if you're trying to target many platforms from one code base. It's decent on most all of them, and was one of the firsts to succeed, but depending on your particular needs, may not be the overall best choice for you.
Small nit, Haxe doesn’t have the upper case X anymore...
One more caveat to add to the end of your note here... Actionscript was probably the biggest standardized language across all web browsers at one time. It’s dead largely because of a business decision, and this left a lot of developers in the lurch. This will undoubtedly happen again.
Haxe is run by a group of developers that insists on keeping control of their tooling and means of development, regardless of what other platforms emerge. This always involves dealing with some rough spots and friction getting things to work initially, but in the long run it pays of with greater flexibility and peace of mind.
Full disclosure - Member of the Haxe compiler team
I'm not sure there's a catch, so much as there's just a number of comparable but corporate-backed alternatives that are seeing some popularity right now:
* TypeScript (Microsoft)
* Dart (Google)
* Kotlin (JetBrains, with Google too)
* Swift (Apple) (though not sure if it compiles to JS)
