Fast, nice looking, cross plattform. Pick any 2.
Though I will concede they put an extraordinary amount of work into hitting all three.
VLC etc: looks terrible.
Sublime: looks surprisingly good. Took a ton of effort though. Perhaps a more appropriate list is
Fast, good looking, cross plattform, reasonable effort: pick any 3.
I've never heard anyone say that VLC or qbittorrent or obs look terrible. Seems like a way to shift the goal posts and have some sort of ephemeral impossible standard of 'looking good'. There are plenty of GUI frameworks that are very fast and perfectly simple to use.
The horrible ones are the custom rendered not-quite-native ones that look like 90’s apps (basically nearly all Gtk, Swing apps and similar). It’s not that they can’t be nicely laid out, productive or snappy they just look horrible in terms of design and polish.
See what I did there?