Sincerely, compared to C++/Qt/WxWidgets, Python/Qt, Swing and even HTML/jQuery Adobe Flex was the most productive environment for frontend and desktop development for me.
It sounds that you did not have a prior experience in Flex otherwise you would know the kinks of the platform. I would say that it was not the choice of Flex that killed your startup but the uniformed choice of a technology that you never had a prior experience with.