Individual Marxists have made errors and weak arguments. But Marxism is dynamic, and it’s strength is in being able to adapt. Marxism accepts the unity of theory and practice. If a theoretical part of Marxism does not work in practice, Marxists revise that part of the theory or abandon it.

For example, if Marx says that ‘primitive accumulation’ happens before capitalism – enclosing the commons and looting the world – someone like Sylvia Federici, Rosa Luxemburg, or David Harvey can criticise Marx and say he was wrong; this method of acumularon does not just happen before capitalism but continues to happen during capitalism. Harvey then reframes the idea and calls it ‘accumulation by dispossession’. Marxists then refer to Harvey’s label unless they find a weakness in it and come up with a yet newer concept to explain their observations.

If Marx fails to fully theorise the state or the tendency of monopolies to multiply and grow, Lenin can come along and write The State and Revolution and Imperialism: The Highest Stage of Capitalism, etc.

This kind of dynamism is familiar to scientists. For instance scientists once believed in phlogiston, but they abandoned that belief when oxygen was discovered. This does not mean the scientific method is inherently flawed, but that it must be developed. Marxism is the most developed form of the scientific method.

Marxists take the same approach to the study of everything, including natural sciences and human society. So there are flaws, but Marxists address and resolve them.

Most obvious flaw is that it is scientific theory, so you have to read a lot. Most people aren’t willing and/or able to do so. Details are also not intuitive (as Marx states himself in book 3 of the Capital), so it overall makes it hard to spread and maintain in a sea of vulgar economic practice that just “works”, not to mention every media average Joe living in DotB is consuming is owned by the bourgeoisie and 100% antimarxist.