There are two main criticisms of interest groups in the United States’ political system.
First, there is the criticism that they, in a sense, corrupt officials. Interest groups are constantly lobbying elected officials. They donate money to their campaigns and to their PACs. Many people worry that the donations allow the interest groups to effectively “buy” the politicians. Even if the politicians are not consciously voting in certain ways based on who donates money to them, they can be influenced by the money. They might only talk to interest groups who donate money to them, which would make it so they only hear the viewpoints of those interest groups.
Second, there is the criticism that interest groups allow the rich to have more power than they should. From this point of view, the successful interest groups are the ones that have a lot of money. They use the money for contributions, but also to pay lobbyists and to mount public relations campaigns. This means that interest groups that have money can have a “louder” voice than interest groups that have less money, even if the poorer interest groups have more members.
Thus, people sometimes criticize interest groups for corrupting public officials and/or for giving the rich more power than they should have based on their numbers.