I LOVE VueJS. But you're right- nowhere near as easy as Flex Builder with AS3.
I built an app with Flex Builder that got me my first job. The application is still in heavy use every day, and I've never even once had someone report that something went wrong with it.
I think long term stability is more an indication of your competence as a software engineer than flex builder's stability.
not bashing flash/flex though, my first job was making educational flash games. I wish I could be as productive and have as much fun when working in html and js.
I built an app with Flex Builder that got me my first job. The application is still in heavy use every day, and I've never even once had someone report that something went wrong with it.