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Pros
Pro Auto resolve dependencies
Any dependencies in the webapp generated can be automatically resolved, thanks to Web-app-generator's powerful integration with Bower.
Pro Image optimizations
Automatic automatic image optimization is a pre-built functionality of the web app generator. So that any sites that are made using this tool are perfectly optimized for performance.
Pro Automatic compilation & linting
As the title suggests, the yeoman watch will auto-compile your JavaScript and Sass scripts for you. Also your code will be run through JsHint (for liniting) to make sure best practices are followed as well.
Pro Built in preview server & live reload
The yeoman watch will compile your source files for you and refresh your page in the browser, as soon as it detects some changes that you've made.
Cons
Con Default support is for libsass
Webapp generator currently has default support for libsass, which falls behind Sass in features (therefor has some incompatibilities with Sass).