Despite the fact that I can set a custom font to a label from Interface Builder and the label even appears with that font when I launch the app on the Apple Watch simulator, Xcode constantly warns me that
Custom fonts are not supported in glance or notification interfaces
Is it just another glitch of Xcode 6.3 or I should not use custom fonts on glances and notification interfaces?
It's not a glitch. Custom fonts are genuinely not supported anywhere except your main WatchKit app.