I’m more curious as to where did you read that the sinicization of Manchus is no longer accepted by historians. Because to the best of my knowledge that is still widely acknowledged and taught. The very fact that by as early as the 18th century many Manchus could no longer speak their native Manchu language (Yongzheng Emperor himself made comments on the need to preserve the Manchu language) is a pretty undeniable evidence.

As to the earlier nomadic conquerors of China, personally I’m not as familiar with the MongolS, but for the Xianbei who controlled northern China from 386-534, they were absolutely sinicized. In fact the Xianbei rulers actively sought to become Chinese themselves. Xiaowen Emperor banned the use of his native Xianbei language and forced the Xianbei aristocracy to speak Chinese and take Chinese names. Other than such linguistic changes, the Xiaowen Reforms (孝文汉化) completely transformed Xianbei society into the Chinese mold—from administrative systems, agriculture and laws etc. In fact, a main cause for the Northern Wei civil war (六镇之乱 Rebellion of the Six Garrisons) was that the northern Xianbei military leaders were upset that the Xianbei rulers in China (“within the Great Wall” as people say) were becoming too Chinese.