Yes, Eric Trump is a liability to his father's campaign because he does not have a professional presence when speaking on behalf of Donald Trump. Eric Trump has made public announcements and media appearances that paint his father in a negative light, often offering conflicting information then what his father's campaign has promoted.
Eric Trump supports his father and backs what he says about workplace harrassment. The outdated notion that victims of workplace harassment are not strong and powerful like his sister, will delight Donald Trump's supporters. He will only be a liability to his father's campaign if he openly criticizes his father's behavior.
Eric Trump seems to appear whenever "massaging" of Donald Trump's comments is needed. Typically in an impromptu interview or by way of a Tweet. If anything, Eric Trump helps the campaign by letting America know what his father Donald "really meant" by his comments. The greatest liability to the Trump campaign is Donald Trump himself.
In response to Donald Trump's comments on sexual harassment, Eric Trump backed up what his father had said. Eric Trump is not a liability for his father. If he were to disagree or make even more outrageous comments than his father, perhaps he could be viewed as a liability. But he supports his dad.