- Almost every possible gtk-3 theme has huge (no, not just big, I really mean the word huge) paddings and there are no working compact variants. Yes there are minwaita and adwaita-slim, but they kinda break Thunar's and Whisker menu's entry boxes, and they also aren't very slick (yes, I don't like proper adwaita either, it does not look very proffessional to me).
So far it meant I had to expand side pane in libreoffice and had less usable screen estate left for editing (the first two pics, notice how in the gtk-3 version the sidepane is not only wider, but also needs a scrollbar!), but now inkscape is getting ported as well, I'd need twice the screen resolution I have now for it to be usable (the second two pics, notice how in the gtk-3 version much more buttons and entries are not directly accessible, and the sidebar is sooo huge and cannot be made smaller) :(
- Scrollbars. They're small, they're ugly, they're hiding and hiding badly, meaning sometimes it's almost impossible to select either last column or last row in a list or both.
Do you still wonder why people are complaining?
Oh, and to end on a positive note: if someone points me to really working really compact theme that does not break xfce-gtk3/libreoffice-gtk3/inkscape-gtk3 a is good looking (no, not adwaita, but something along the lines of numix, greybird, menta, zuki*, etc.) I'd be very happy to change my mind about gtk3 ;-)