What is the main critique of pluralism & how do those who support it counter that argument?
I assume that you are talking about pluralism of the sort that Robert Dahl wrote about in Who Governs? This is the idea that the American political system is open to any group that wishes to compete in it. There is no elite that con dominate the system. Instead, there are changing coalitions of groups and various groups tend to win on issues that they are most concerned about.
The main critique of this idea is called elitism. It is the idea that there is indeed an elite that does control the system. C. Wright Mills imagined this as an actual small group of upper-class people who were tied together by things like having gone to the same sorts of schools. However, elitism can also hold that those with money dominate the American system. In this view, groups with money can overpower the groups that Dahl believes in. Dahl believes that it is the number of people in a group, and their degree of fervor on an issue, that will determine if that group can get its way. Elitists argue that groups with money will defeat the groups that only have numbers and fervor.
The elitists, then, argue that the pluralists are wrong because the pluralists underestimate the power of money in our political system.