All praise is due to Allah. May Allah's peace and blessings be upon the Messenger of Allah, his family, and companions.
I have not read the words of Dr. Wahbah az-Zuhayli, and I do not have his book, so I cannot cross-check. However, the scholars of usool al-fiqh and other disciplines discussed this issue on the discussion of the qualities of a person whose opinion is considered when arriving at a consensus. The opinion that was held by the majority of the jurists and the scholars of usool al-fiqh is that in reaching a consensus the opinion of a immoral person is not accepted at all whether this immorality is in the area of belief or actions. A number of scholars stated that an example of the people who are immoral in belief are the Shias. Al-Maawardi (may Allah have mercy on him) said in at-Tahbeer Sharh at-Tahreer 4/1560, "The opinion of an immoral person is not considered at all, regardless of whether that immorality is in the area of belief or actions. An example of people who are immoral in belief are the Rawaafid (the Shia), the Mu'tazilites and other sects."
Ibn al-Qattaan said, "In my view the consensus to be considered is that of the scholars. As for the people who follow their desires, then they have no way to be part of the consensus." He stated this in al-Bahr al-Muheet 4/468.
My view is that their difference of opinion to the Ahlu Sunnah in the principle of evidence is not considered because they do not consider the consensus of the scholars, and they do not regard it as evidence.
The author of al-Ibhaaj said, "I have previously mentioned that the Shias do not consider the consensus of the scholars upon which the mujtahids of the ummah have agreed and regarded it to be a proof." All this concerns secondary issues. As for issues of belief, the Shias are not considered, otherwise the Ahlu Sunnah would not have agreed upon anything. And Allah knows best.