Were the conquests of some Umayyad and Abbasid caliphs in accordance with Islamic mercy, or were they unjust to non-Muslim lands, and what about the conquest of Al-Andalus?

The existence of injustices during the Umayyad era is undeniable. These injustices befell both Muslims and dhimmis and were one of the reasons for the demise of the Umayyad state, for oppression hastens the destruction of nations and states. Another reason was the Umayyads' failure to establish a civilizational movement that kept pace with the missionary expansion after the conquests. Nevertheless, these injustices are minor compared to the prevailing situation among non-Muslims. The Umayyads dealt with the people of the conquered lands with tolerance, kindness, and humanity, as mentioned by historians like Will Durant.