I heard Engels gets some things wrong like applying dialectical materialism to the natural sciences (which Marx didn’t agree with)

From the preface to the second edition: "I must note in passing that inasmuch as the mode of outlook expounded in this book was founded and developed in far greater measure by Marx, and only to an insignificant degree by myself, it was self-understood between us that this exposition of mine should not be issued without his knowledge. I read the whole manuscript to him before it was printed, and the tenth chapter of the part on economics ('From Kritische Geschichte') was written by Marx but unfortunately had to be shortened somewhat by me for purely external reasons. As a matter of fact, we had always been accustomed to help each other out in special subjects."

When Marx and Engels Disagreed, Ian Angus: "If, as some critics claim, Marx rejected the application of dialectics to non-human nature, Engels would likely have expected and received sharp criticism-for Marx was never reluctant to argue. But Marx did not object, nor did he assume that his own understanding of dialectics qualified him to judge Engels's thoughts on natural science. Instead, he replied that he had learned much from Engels's letter (it 'edified me greatly') but would 'venture no judgment until I have had time to reflect on the matter and consult the 'authorities.'' The 'authorities,' of course, meant Carl Schorlemmer-on such matters, Marx deferred to a professional. His letter concludes: 'Schorlemmer read your letter and says that he is essentially in agreement with you but reserves his judgment on points of detail.'"