This extension is not submitting wpUnicodeCheck and therefore all attempts to save comments using it will fail. Also, it is adding an edit token after the comment form has been submitted rather than adding it inside a hidden element in the form beforehand. And the extension has not had real updates without dependency or localisation updates in months. As such, I am not very confident about this extension.
Improvements have been merged: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Commentbox/+/654567
At first glance, this is a low risk extension. It even rejects external links since there's no support for https://www.mediawiki.org/wiki/Extension:SpamBlacklist. Seems fine to enable, what do you think?