That the Court gives vertical but no horizontal direct effect to directives is argued to may lead to discrimination. Why?

"[I]t is significant that the CJEU has applied the principle [of direct effect of directives] solely in cases between citizen and Member State, and then only when the directive was for the citizen's benefit and not to their detriment [...] (known as 'vertical direct effect'). The direct effect of directives in relations between citizens themselves ('horizontal direct effect') has not been accepted by the CJEU."