Speed is what you're missing on. Sketch was designed using modern, native libraries and it doesn't come with all the stuff PS has (3D, video editing, 90s style shape styles and filters, some print stuff, some painting stuff) - it's just for UI and it feels like a breath of fresh air to use it.

The lack of a windows version and low market share means it's not that easy to use for professional projects and it does have its share of bugs and problems, but it's damn fast and easy to work with.

You should try it when you have the time.