How can a "crayon" license be a problem?

Bruce Perens (co-founder of OSI) has coined the term "crayon license":

I've been calling these "crayon licenses", taking a line from an old Monty Python sketch about a dog license with the word "dog" crossed out and "cat" written in, in crayon.

The licenses referenced are typical well-intended free software licenses that have been written by programmers in simple, easy-to-understand, non-legalese language. Some typical examples are the WTFPL and the somewhat more elaborate Artistic License 1.0.

Perens seems to imply that such licenses may pose a problem for developers using them, or for the open source community.

How can these licenses be a problem?

Or is Perens wrong about them being a problem?

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