To what extent is what is stated in Al-Mas'udi's "Muruj al-Dhahab" and Ibn al-Athir's "Al-Kamil fi al-Tarikh" regarding the distortion of the conflict between Ali and Mu'awiyah (may Allah be pleased with them both) accurate?

Shaykh al-Islam Ibn Taymiyyah warned against al-Mas'udi's book "Muruj al-Dhahab" due to the abundance of falsehoods within it and because al-Mas'udi was known for his Shi'ism and Mu'tazilism. Many scholars supported Ibn Taymiyyah in this regard. As for Ibn al-Athir's "al-Kamil fi al-Tarikh," it contains a Shi'i inclination, even though Ibn al-Athir was not a Shi'i. Many Sunnis have praised it, yet it contains certain matters that require caution, including: his stance on the 'Ubaydid state, his stance on the narratives of discord during the era of the Companions, and his sympathy for some enemies of the Companions. It is advised to refer to the books of Sunni scholars known for the soundness of their creed.