Flex was better than anything in use for web front end to this day, imo. Aside from requiring Flash, but iirc targeting HTML/js as an output format was on the roadmap.
We have Apache Royale https://royale.apache.org/ for that. Compile your Flex MXML + ActionScript to HTML + JS and more.
Yet, as a user, there's nothing I dreaded more than the all-Flash application. Binary blob, no integration with the OS, and accessibility was an afterthought. Basic things like highlighting text to copy, tabbing between controls, Enter to submit, would not work. We should take care to not view the history of Flash through rose-tinted glasses.