I think that if you develop your application from scratch and directly use Redis as it was intended (data structures over the network) then it makes sense. I've done it for some applications and it's quite nice.

But you have to make the choice to skip using a relational database, and a lot of application frameworks make it very easy to use a relational database out of the box, to the point where you would have to make a conscious choice to use Redis directly, and sometimes for a CRUD app it's easier to just use the RDBMS.