They're probably referring to the functional regressions it made with it's first official release, and how it furthers the idea of GNOME/GTK lock-in. libadwaita has made it extremely difficult to package cross-platform desktop apps, especially while appearing native on different desktops (eg. adopting the native Breeze look on KDE while retaining the native Adwaita look on GNOME). The lack of this functionality at launch (and subsequent empty promises of a replacement) have rightfully left a bad taste in some people's mouth, particularly now that much of the GNOME leadership denies that this is a problem in the first place.