Why do doorway pages cause problems?

As the creation of doorway pages is very simple, they are used by many website operators.

However, search engines have quickly realized that they are spammed by doorway pages. This leads to search results that often only consist of doorway pages. These search results, however, reduce the value of search engines and are of no interest for search engines users. Therefore, search engines run the risk to lose visitors, if they do not pay attention to the quality of their search results.

This is the reason why search engines took steps against doorway pages. For example, AltaVista, removed doorway pages from its index, and black-listed some websites that had used doorway pages. Other search engines did the same or will do the same. That is why you can act on the assumption that doorway pages involve more risks than advantages.