Drush 8x working from directories above web root, but not in web root
Fixed: we figured out that composer and drush were using an older version of PHP which wouldn't work w/ Drupal 8. I still don't understand how the webserver can run one version (phpinfo.php showed the site running the expected 5.6 version) while Drush is set to another. Anyway, finding and aliasing to the proper version fixed the issue. The fix was adding this to .bashrc: (your path might be different). 
    alias php='/opt/php56/bin/php'
    alias composer='/opt/php56/bin/php ~/bin/composer'
    export DRUSH_PHP='/opt/php56/bin/php'