One of the biggest problems with "mixed systems" is when they allow private wealth to steal public wealth. Private wealth is a threat to the public, because it's power, so if a few people amass more power than the rest, that destroys democracy and lessens the power of certain people in society.

One of the fixes is to democratize businesses. You could also disallow profits and require fair wages for all businesses. There are absolutely problems with top-down planned economic models, and the solution is always more democracy, because the system should always be working to meet the needs of the people, and not the other way around. That's how you get freedom, is when a government is barred from preventing freedom, and much better when it's not adversarial and is actively working to meet everyone's needs as it should as that is the entire point of government, to serve the public.