I know and test those values also.
Interestingly enough they got worse due to the updates of the webkit engine by Jolla.
So the browser depends on the qt5 version and its qtwebkit version shipped with SailfishOS.
I hope that a qt5 update will bring in better support but effetively qtwebkit development is on standby and only fixes are coming in as they are switching to chromiums blink engine with qtwebengine in future releases. HTML5 Audio & Video was disabled by jolla as they weren't working really (which to the extend I understand it is a problem that there is/was no gstreamer integration which is the default media processing engine on Sailfish)
So I can't promise or do anything in that regard besides waiting and then porting to qtwebengine when it is available.

Youtube-dl does the same thing as Webcat do basically to get the video information and to download it.
My guess is that the qt libraries I use for the download manager are creating the problems. Therefor I integrated curl downloading.
But I am not so sure myself yet.
I am still investigating and I want to keep the dependencies minimal (which means no youtube-dl for now)
I rather like fixing my own code before I incorporate any other