I thought flex was great. I really did. The funny thing is I used to do a bunch of cold fusion work back in the day so I already had firsthand experience with Adobe's complete lack of interest in supporting a developer platform. All I have to say is "allaire homesite", most CFdevs should remember.
Nevertheless I got involved a handful of huge flex applications. This was back when it was Apollo, so 07/08. Firefox was gaining traction but not much, Jquery didn't really exist and google was for searching, not web browsing.
These apps looked beautiful, every pixel was accounted for, they were responsive(ish) and, the best part, when they were done they were DONE (mostly). No more trying to support IE5.5 or IE6. Flex was something you could sell to an enterprise shop.
Then native technologies finally caught up. The IPhone, perhaps rightly so, gave a huge middle finger to Adobe. Then JQuery, then Bootstrap comes into the picture and... amazingly... the shit actually works. Seriously, this is not expected behavior when dealing with html/css/js libraries.
Flex was really just a means to an end. For devs like me it meant I didn't have to code another $#@! table layout or drop-down-select or menu-navigation or accordion.
On one hand I feel the industry caught up to Flex and the better player (native support) won. I feel that I shouldn't personally blame Adobe on this one. On the other hand I know that Adobe has killed off so much through their process of atrophy-driven-development that I can't help but want to point a (very specific) finger at them.
Eh, yeah, at this point when I hear Adobe I walk away, I can't blame them for everything, but I've lost all faith in their ability to deliver.