A core tenant of the theory of capitalism and a free market is competition. However, in reality, capitalism eliminates competition by allowing monopolies. Monopolies form by crushing small companies or through mergers and acquisitions. This decreases competition and harms consumers' rights by letting those few corporations set price and quality standards.
They also have excessive political influence through campaign donations, lobbying, and the propaganda they can create.
Capitalism is also unstable. Monopolies create instability. There are now corporations that are “too big to fail”. This means they are so big that their failure would be a disaster for the entire economy. This is an unstable system. During the global recession from 2007-2009, banks had to be bailed out because they were too big to fail.
This results in a system where corporations receive socialist assistance with government regulations, subsidies, and bailouts. As Martin Luther King Jr. said about America, “we all too often have socialism for the rich and rugged free enterprise capitalism for the poor.” 
This is the reality of capitalism.
Another reason that capitalism is unstable is because of its economic cycles. Capitalism creates periods of great growth (booms) and busts. Unfortunately, most people can't save enough during the booms to adequately save for the busts. For example, in the U.S. 78% of people are living paycheck to paycheck, this means they aren't saving 
These busts lead to recessions or depressions and massive unemployment. They are incredibly harmful to people.