First we must pay attention that neither the Twelver Shia consider other Islamic sects as kafirs, nor do the majority of the Sunnis accuse the Twelver Shia of kufr, and if it is ever said so, it is by small extremist groups such as the Wahabis who hold no significant status amongst Muslims, not even the Sunnis.
Twelver Shias, as a result of their strong ability of argumentation and great scholars, have backed and confirmed their beliefs through sources that those opposing the Shia see as authentic and have also challenged the beliefs of others, thus becoming subject to the animosity and anger of extremists. At the same time, such a high ability of scholarly dialogue and level of confidence is devoid of in other Shia sects such as the Isma’iliyyah, not making them threats to extremists. Consequently, hostilities are much less here and efforts are made to prevent their assimilation into the majority of the Shia (which are Twelvers) through appropriate attitudes towards them.