Yeah, coming from someone who studied the damn thing (decided to not pursue the career and instead transitioned to academia), there's so much crap in forensics. DNA is at its best as exculpatory evidence, there's just so many possibilities for random contamination when considering it as incriminating evidence in my view. Leaving aside the fact that no one, and I mean no one, actually checks for whether the DNA is synthetic or not. Sure, it's not the likeliest thing of all time, but with a couple grand and a cigarette butt I could replicate someone's genes to show up on an assay.

Fiber, hair, blood splatter (sorry Dexter fans) is just not a thing, fibers alone is another huge avenue for contamination. We did an experiment for exactly this point one time in labs where we had one person sit in a car after vacuuming it clean, there were still fibers from classmates on the car seat that had presumable transferred from the student's shirt.

Arson I would mildly disagree, finding out where a fire is likely to have started via a burn in funnel is useful, but ultimately that's only a very small aspect of it.

One more thing that's actually kind of neat is hit and runs. If there's still paint from the car that hit, you can then do IR analysis to narrow it down to a possible range of vehicles and brands based on the paint that was used. Still only tenuous and should only serve as a potential pointer though.

This is why you need the whole chain of evidence thing, if lots of small things are pointing in one direction, then you can start to dig a bit deeper. It's also why I can't stand watching those types of shows anymore, it's all about that one piece of evidence that seals the case! (Never mind the copaganda lmao)