The best way to get reliable facts are from the gwt survey. One of the biggest issues with GWT has always been a long compile time. Fortunately, it's improving very quickly so it won't be a significant issue in the near future. Another pitfall is that GWT is dramatically more complicated because Java is a more complicated language that resists bad coders every step of the way. In addition, compiling adds a layer. For example, js interop requires a little boilerplate. The fundamental issue is that GWT wasn't designed to be simple. It was designed from the ground up for extremely complicated web apps and the entire community consistently prioritizes, performance, code quality, architecture etcetera over easy coding.
Remember that you can use js in GWT at any point so if you are struggling with GWT consider using js. At the end of the day GWT is js so you can do anything in GWT that you can in js. In fact, most GWT projects use js. The problem is that GWT is drastically more complicated. Nevertheless, it's sometimes worth the extra complexity.
It's worth noting that GWT 3.0 will bring massive improvements.
