What viewers are best for multiscreen coverage: HTML5 viewers VS Universal viewers?
Adobe recommends that you use HTML5 viewers (OOTB viewers or SDK) for multiscreen coverage. For example, the HTML5 fly-out viewer is certified to work across desktop and mobile using the single viewer.
Universal Viewer is only recommended if you want to use different viewers for different screens. That is, if you want to use Flash on desktop and HTML5 on mobile, for example. Or, you want to use Zoom with buttons on desktop and fly-out for mobile, and so on.