Is voter suppression effective?
Are qualified voters being kept from determining the outcome of elections?
I don't feel competent to answer the question as asked. I have no personal knowledge of voter suppression at the polls and cannot judge its effectiveness.
However, in a broader sense, voters are suppressed in the United States because they have no mechanism by which they can proclaim their own political choices. This form of voter suppression is exceptionally effective.
In America, the issues and candidates the people are allowed to vote for are controlled by political parties, and a party-based political system is profoundly undemocratic. It expresses the people's status as subjects of those who define the options they may vote for; they are subjects of those who control the political parties. As long as parties control the choices on which the electorate is allowed to vote, the people are helpless because 'those who control the options control the outcome'.
In such a political environment the question of whether or not qualified voters are "being kept from determining the outcome of elections" is moot because they have no effective participation in the selection of the choices on which they vote.
Until qualified voters have a way to participate in the selection of their representatives in government, their participation in maintaining the existing power structure can have no validity. Until we enact an effective means for the entire electorate to participate in the selection of issues and candidates, to the full extent of each individual's desire and ability, we cannot stop the immense financial interests that control our political process from plundering us and our environment.