Not actually true. This is not to say that courts get it right all the time, but it's not particularly newsworthy when they do. Attacking the chain of custody and alleging contamination or false positives are standard techniques in challenging forensic testimony, both biological and digital.

The cases where the forensic evidence is legitimately challenged and those challenges are ignored or overruled are often dramatic and newsworthy, but the rate of miscarriages of justice is falling because courts today are much more aware of these issues than the courts of (say) 20 years ago. Unfortunately news reporting of trials and appeals is so utterly awful that people tend to assume the exceptional is the norm.

I'm not saying that it isn't a problem, just that the courts are more cognizant of the issue than more people appreciate.