I hit a serious, site crashing issue on a brand new 3.9 site. Changing to the WP default theme and disabling all other plugins both did nothing. The Yoast plugin, when activated causes a redirect loop.
The product itself (until this) is of course really slick. But when I purchased the premium support, a junior dev gave me a general answer and even said "we've never see that before" and then they vanished.
Completely let me down on a new site...I'm disappointed.