Ranked in these QuestionsQuestion Ranking
Pro Has both GTK2 and GTK 3 support
Users will be able to match their gtk2 and gtk3 themes easily due to Arc supporting both.
Pro Really clean and nice-looking
Pro Supports gnome-shell
Pro Customizable to dark or light themes
Pro Available firefox and chromium themes
There are matching themes for Firefox and Chromium for those that want to have a fully matched looking environment.
Pro Actively maintained
Actively maintained, support for it won't be going anywhere anytime soon.
Con Has some contrast issues
The theme is a low contrast theme so it really depends on the screen brightness if you see borders or the text, some elements don't even have borders - this makes the theme barely usable for portable devices.
Con Bleak font colors that cause eye strain
The font color in Arc and Arc-Darker is not black, but lightgray
after prolonged usage, it becomes apparent that this font color, combined with white backgrounds causes too much eye strain
Con Close button is not activated when cursor is moved to the top edge of the screen
The simplest of all ergonomic design principles is not followed.