Libadwaita is missing one thing:
Support for third party style sheets.
However there is a nice project on github theme changer for Libadwaita that works.
Of course its possible to change the scss code to look very different. Its as much stable as early gtk3 versions. And it supports to load css files very well.
Why the hell did the gnome team make the not understandable decision to force a certain rather flat design to “normal” users, while it is possible to load other styles as well? wouldnt it be better to include the theming part in a special library that can be modded like theming engines in gtk2?