I like that it is easy to use, understand and can be used in any programming language you want to work with.
At least initially it was difficult to install packages or run programs because of dependencies issues but this has been improved recently. It's still not as user friendly than Python but the community around shiny support is very active so hopefully they will get there soon! We are using shiny to create an interactive website where users can submit their data easily and have all results automatically displayed without them having to write code themselves.