A Brady violation is one of the stronger grounds for appeal that exists in criminal law, but strong does not mean automatic, and the road from violation to overturn is long and difficult.
Brady v. Maryland is the 1963 Supreme Court decision that requires prosecutors to disclose any evidence that is favorable to the defendant and material to guilt or sentencing. That includes exculpatory evidence that could prove innocence and impeachment evidence that could undermine the credibility of witnesses. When the state withholds that material, it is a constitutional violation of the defendant's due process rights.
The catch is that proving a Brady violation on appeal requires showing three things. First, that the evidence was actually suppressed by the prosecution. Second, that the evidence was favorable to the defense. Third, and most importantly, that the suppressed evidence was material, meaning there is a reasonable probability that the outcome of the trial would have been different if the jury had seen it. That third element is where most Brady appeals run into difficulty. Courts apply a high standard for materiality, and judges are reluctant to overturn verdicts without clear evidence that the withheld material would have changed the result.
If a Brady violation can be documented and the suppressed evidence is genuinely material to the outcome, it is one of the better appellate arguments available. But as with all appeals, getting the claim heard and prevailing on it are two very different things. This requires an attorney who knows appellate procedure and has experience arguing constitutional violations at that level. A general practice criminal defense lawyer is not the right fit for this work.