WHich was... only windows computers since Flash barely worked on Macs or Linux, and never properly made it to mobile, where it was also a complete battery drain when it did in fact do things... sometimes.
It took us good three years, it was eventually feature complete and used by 120 people on two local installations synchronised back then with Dropbox (hey, I didn’t know any better and it actually worked great over a hurricane m). We have replaced a home-grown COBOL app.
Browser plugins could have been fixed to catch up, the VM itself wasn't the problem in terms of drain
Adobe Flex was interesting and not awful. The problem was the runtime.
And the company behind it. Who remains awful.