I notice that when I set an OpenType feature with the selector at the lower left of the edit window (say, liga), the preview and edit panes in the app display the substitutions triggered by the feature right away, but the Glyphs Viewer on my iPhone doesn’t update immediately. If I change the text (even just arrowing one character over) the iOS text updates, but I would suggest if possible that a message to update the text in Glyphs Viewer be triggered by the feature selection.
One more suggestion – line breaks. =)