It is a problem because no one else actually managed to properly fill in the many gaping holes.
Which holes though?
Again, there's an assumption (which you see on both the left and the right for very different reasons) that everything Marx did was in service to this concrete political agenda called "communism" which has incredibly specific objectives and yet somehow, mysteriously, people just can't gosh-darn well understand because their brains are small or because Marx, despite
clearly having this elaborate 4D chess plan, somehow didn't bother to explain it. That assumption is wrong.
Every single greater scale Communist experiment failed because it was built upon an unfinished concept.
What does failure mean in this context?
It's an answer that probably seems very obvious but deserves to be investigated. There are still communist countries around today. China is a emerging global superpower. When exactly did the Chinese communist "experiment" fail?
In fact, let's ask a really dumb question noone would generally think to ask. Has liberalism failed? Is liberalism an unfinished concept?
Of course not, right. Many successful societies are explicitly based on liberal principles. The US is the richest and best country on earth and its foundational documents are all about the liberty and how the legitimacy of government is based on representation of the popular will and democracy and shit. But think about it. Has liberalism actually delivered any of the things the radical liberals of the Enlightenment said it would? The French revolution failed. The American revolution was carried out by people who owned slaves, and has created a country so transparently hypocritical in its approach to liberty that a demographically significant proportion of its population think their leaders are pedophiles who worship Satan.
If anyone bothered to read Rousseau or Locke or Voltaire instead of spending their school lives being fed circular nonsense about how lucky they are to be born in freedomland where everyone is free because freedomland is the best country where everyone is free because they live in freedomland, would they actually recognize the society they live in as being the fulfilment of the utopian dreams of the radical enlightenment?
To be honest, probably not.
Every single greater scale liberal experiment failed because it was built on an unfinished concept. All concepts in political thought are unfinished because (unless you're Francis Fukuyama and willing to publicly humiliate yourself by posting neoliberal cringe) history isn't finished, and we have to live in it anyway. Liberalism didn't give us the utopian society everyone creamed their stupid knee-britches over in the Parisian salon. It didn't usher in a new age of Enlightenment in which every man, imbued with the natural inheritance of reason, became a self-governing individual free from the decrepit trappings of custom, bound by voluntary contract into a greater commonwealth of free individuals animated by the golden light of liberty and justice. It gave us this shitty, banal world we live in where people starve to death while one incredibly stupid billionaire spends more money than any of us will ever see making sure his tweets get the most views.
People died for this. People willingly offered up their lives for a better world, and this is the one they got. What word could possibly describe that other than failure?
Marx' understanding of history and especially feudalism is pretty poor by todays standard.
Is it wrong though?
Like, that little branch of social theory which exists at the intersection of philosophy and history often gets picked on as a soft target by "real" historians looking to show off their massive throbbing primary sources, but by my reckoning these attempts frequently end up missing the point.
Marx isn't writing about feudalism to help his readers gain a detailed understanding of feudalism because god damn it I spent a decade learning middle English instead of getting laid and now you all have to suffer. The question is, is the understanding of feudalism sufficient and accurate enough to facilitate a deeper understanding of the present moment?