One thing to be aware of is that if you move it to the root (or rename directory) and this is an established forum (e.g. you have imported from bulletin) then there can be a problem if you decide you want redirects, because if you redirect form the /forums/ directory you get complications because of /forums/ being a necessary part of the Xenforo URL structure when you are viewing the forum list or thread list (forum view).
You can change any existing internal links but any existing external links that would have /forums/ would have this issue.
There may be a way round it with route filters, for example you can rename /forums/ so it has something else in the URL, but I don't think you can remove it.