One obvious issue with both these downloaders is lack of proper modularization. Which greatly hinders adoption, I believe. I would expect some kind of plugin system. Naturally, there are attempts to fix that:
https://github.com/un-def/dl-plus is one example. As a bonus that would help greatly with recent youtube-dl sort of situations and RIAA would have barely made a splash.
Plenty of other features immediately come to mind as well: universal media support, proper parallelism, GUI, desktop integrations, proxies support, anti-captcha... The punchline is: all of this and much more you could find in jdownloader 10 years ago. But somehow youtube-dl won that race. How did that happen?