Each template has built-in settings that allow for multiple different layout options, choice of which elements should be shown or hidden and change things like the background image all without having to edit the template itself.
Though Apple now tries to converge its macOS software packages to iOS markets ("click here to buy"), Keynote still excels in the area "I want to do xxx, how to achieve?". Using Keynote, even a user who has never done a presentation before, will produce remarkable presentations, provided he's willing to invest a certain amount of time, that is less than half of the endeavor needed to create a slick Powerpoint presentation.
Prezi helps you create presentations that tell a story. The UI lets you create a storyboard for the presentation so it's really easy to design the big picture for the presentation and then drill down to the individual "slides".
Instead of being constrained by individual slides, Prezi lets you input all the various text & media you want to cover and then organise it into a logical flow later on.
Adding too many animations might cause it to slow down quite significantly, as well as on older versions more complicated animations might not work very well.