Hard link '~/Library/Mobile\ Documents/com~apple~CloudDocs' to ~/iCloud
ln(1) on OS X does not support folder hard links. A symlink would work fine instead.

ln -s /Users/Rowan/Library/Mobile\ Documents/com~apple~CloudDocs/Documents /Users/Rowan/iCloud


If you really want a hard link, you can install coreutils with Homebrew and use

gln -d /Users/Rowan/Library/Mobile\ Documents/com~apple~CloudDocs/Documents /Users/Rowan/iCloud