So basically you can resize a subwindow, big deal. In a properly designed interface that shouldn't be necessary. As for displaying varying amounts of information again that goes back to the dev. Typically that sort of option is referenced as verbose level, useful to have different levels in logging situations but in a game if you can't find the right level for 99.99999999999999% of potential players you're just a bad dev.
As far as screen management goes they've virtually the same. They both use the same basic property set and include the option to define styles (CSS classes) to be used on multiple objects by reference. Admittedly you can float shit easier in html/CSS but given the frankly amateurish interfaces common to most indie projects let alone QSP titles, which offer about as much visual complexity as twine or rags, you might as well just do them as pure html and save people the time getting a player.
Just cause few choose to use it among the low end patreon amateur dev community doesn't make it moot, it's still a capability that QSP does not have. Look at the larger paid projects like those you'll find on stream and they tend to be used more.
AVCodec is x264, like I said VP9 is x265 equivalent so better quality to size ratio available in RenPy. The fact remains the RenPy devs intended the use of video or they wouldn't have given the option while it took the QSP fan community to bring that support to the platform cause the original dev felt it unnecessary, undesirable, or was just too damn lazy.
As far as security goes anything you can make a browser do you can make QSP do because that's basically what a QSP player is, a halfassed web browser with no security of any kind. Now you do make a point that you can put raw python into RenPy scripts but given RenPy uses it's own proprietary script set which is compiled by their code at runtime the platform can ensure a certainly level of protection against untoward actions.
Anyway RenPy was made to support anything those who use it might want including a complete graphical interface with all the bells and whistles, music, sound FX, voice acting, videos, frame based animations without having to resort to crappy animated gifs, etc... while QSP was designed for those who wanted effectively iframes they could resize and rearrange.