I know drupal has changed significantly since i've worked with it... I was accustomed to working with it fairly similar to how i interact with wordpress.  Now that it has a symfony stack it's significantly different, with wordpress it needs to be within the webroot and my standard implementation has the root composer.json above the wordpress directory.  However, in that case it may be getting into handling webroot packages which i believe is outside of the desired scope of this project.  I created a separate installer for that use case, and i have PR #52 to take care of the ability to manipulate where the project lives in the root composer.json.  Maybe implementation of child directory installations would not be necessary with that PR being merged in.