I mainly work on in-house applications - cool stuff (by virtue of the companies I've worked for), but rarely anything seen outside of the firewall. Working in creative environments means UI/UX is paramount to the success of a project.
I've built frontends for these apps in both Flex and using various JavaScript libraries (mainly jQuery). I have no idea what you mean by "they would never scale" - unless you're stuck supporting older versions of IE, I'm hard pressed to imagine a Web-based UI that would require more umph than modern JavaScript interpreters can deliver, but it is possible - I have a number of backend data management tools for a few projects, which I use to manipulate large datasets, and even current releases of Firefox is crushed by the jQuery work I'm doing, though Chrome and Safari both are snappy. For the frontends, I'm fortunate not to have to support IE, so I target Firefox as the minimum platform (it's currently the slowest JS interpreter I support).
As I mentioned, I also used to do a lot of Flex work - frankly, I loved Flex, and have little negative to say about it. It's everything I wanted from Flash technology as a developer - a consistent runtime... a pretty homogeneous development environment, decent UI libraries, solid database connectivity, easy path to compiling "native" apps with AIR... that said, since getting interested in jQuery I rarely, if ever, have a desire to go back. I'm not sure the choice between the two was purely rational, but I find myself longing for Flex much less now doing work that runs on a browser engine than I longed for HTML/JavaScript/CSS when I was working in Flex.
But again, what do you mean by "they would never scale"? Plenty of massive apps use JavaScript on the frontend... yes, older versions of IE suck terribly as a JavaScript runtime, and even Firefox has it's performance limits. If you have to support IE, and are building internal apps, I highly recommend Flex. If you don't have to support IE, just take your pick - learn both. If your UI is so heavy it's crushing your JavaScript interpreter, you're likely doing something very wrong.