<p>In which parts of a project writing unit tests is nearly or really impossible? Data access? ftp?</p>

<p>If there is an answer to this question then %100 coverage is a myth, isn't it?</p>

Data access is possible because you can set up a test database.
Generally the 'untestable' stuff is FTP, email and so forth. However, they are generally framework classes which you can rely on and therefore do not need to test if you hide them behind an abstraction.
Also, 100% code coverage is not enough on its own.