I can't really tell how JSBuild works because the docs aren't very helpful and I don't feel like diving into the code right now, but I can tell you RequireJS only supports modules in one of the wrapped "transport" / "async module definition" formats, NOT all CommonJS modules. The author is very opinionated about this so I wouldn't expect that to change any time soon.
The basic difference between JSBuild and the other alternatives is that JSBuild generates unobtrusive code for specified CommonJS packages or apps. You may check the "How It Works" section in thee user guide for details.