Learning how Yesod works internally is hard. It is a large framework with complicated components. Finding the appropriate code and understanding how it contributes to the framework is difficult, meaning developers will struggle.
There is extensive documentation and tutorials for Happstack and Happstack Lite available for use. Documentation and tutorials help programmers write their code; with so many options, programmers will have an easy time learning the framework.
When an exception occurs on the server, the error gets printed. By seeing an error that makes no sense to the user, his experience is negatively impacted. Having a negative experience can mean a user never visits the site again.
The comprehensive Snap documentation is currently outdated (last activity appears to have been 2013), though Snap itself is under active development. Note that this primarily affects tutorials and guides; the API documentation is generated automatically and remains current.