Colin Wilson offers the arguments against Foucault from a Marxist perspective:
Foucault’s ideas are radical but not Marxist, which helps explain his current prominence. His ideas fit a time when Marxists are a minority and non-Marxist radicals such as Noam Chomsky are greatly respected.
Foucault’s approach leads to some serious problems. He rightly rejected both capitalism and Stalinism. This was a necessary first step, but he never explained how ideas relate to the material reality of society – where ideas come from, or what role radical ideas can play in changing the world.
The closest he comes to an overall explanation of society is in his writings from the 1980s about “power”.
By “power” Foucault means all non-economic forms of social domination. But he describes power as existing everywhere – so you cannot say that one group of people has it and another does not. Foucault was right to stress the importance of non-economic factors, but his explanation is too vague to be useful.
There are serious political weaknesses in Foucault’s work. But many people are inspired by the radical side of his writing. He may not be easy to read – but what he does have to say is almost always thought provoking.