Typically, the "problem" is capitalism is that someone who wants a thing will buy it with so much money you want to accept the money so you sell it to them willingly, or someone owns a thing and will not part with it for an amount of money you can or want to spend on it.

Power accumulating in one person or group is a major problem, but that occurs under every economic system. Under Feudalism, the King owns literally everything, and if the King has a best buddy then that best buddy may end up controlling large swaths of everything. Under command economies, the central government controls literally everything, and the people who are in good sway with that government will do well and those who are in bad sway with that government will do poorly. Under tribal communism economies, the chief and elders control literally everything, so those people can ensure their families and friends are better off than their enemies. Inequality is unfortunately a state of nature. Witness our own solar system, where as far as we know, one and only one planet ended up with 100% of the life.

There has never been an economic system implemented at a national level that does not have haves and have-nots, and that does not have some measure of the powerful and some measure of the powerless.