Testing is easier with both, than it is with Web API / MVC. Both are faster than Web API / MVC. Routing is far superior in Nancy than MVC / Web API. Content Negotiation is far superior in Nancy/ServiceStack than it is in WebAPI.

I could do this all day. You end up fighting the framework using MVC / WebAPI, once you build an app or service with Nancy or ServiceStack, you will be like "wow why was I using the MS stuff so long..."