Validation using custom scalar types

From your video [MERN Stack & GraphQL - #7 Server-Side Validation](https://www.youtube.com/watch?v=t8qpxmuq8CQ), it seems that you don't approach the subject of using custom scalar types for [validation](https://www.apollographql.com/docs/graphql-tools/scalars.html#Validations).

There are already several packages that use that idea:
- [@okgrow/graphql-scalars](https://github.com/okgrow/graphql-scalars)
- [GraphQL Custom Types](https://github.com/stylesuxx/graphql-custom-types)
- ...

What are your thought about it?