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Signatures are the tip of the iceberg. Plenty of other forms of bs forensics live on in the legal system in some shape or form. e.g., fingerprint analysis, polygraphs, field sobriety tests, devices that literally do nothing, trainings on reading facial expressions, and so on. If you can take a two week course on it, then chances are there is some cop somewhere using it to detain people.

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fingerprints seem pretty solid?

On a good day, with an excellent print and the best people doing the matching you can be pretty sure if this print is from this person, but crime scenes are not that perfect scenario and the actual crime scene investigator might be less than great at the matching or influenced to some extent by what their boss wants.

There's a big difference between "This thumbprint in blood was on the recovered murder weapon and it's a perfect match" and "This smudge of half a finger on a paper bag found near the scene was arguably a match" but the jury isn't necessarily told about this and where on that scale the evidence they've been told about would lie.

Standards vary wildly on what constitutes a fingerprint match. There can be well over 100 ridge characteristics in a fingerprint, but some US jurisdictions only require as few as 12 of them to match, and it all comes down to an investigator's subjective determination anyways. It is not scientific.

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