How so? You might be surprised if you were to study the subject. Lawyers know how to make everyhing seem fuzzy. If you are judging forensic entomology by what you saw going on in court in this one case, you are making a mistake. The entomologist from Hawaii was stretching things, big time, and ended up looking quite awkward and probably hurting himself professionally.
Because you can take someones matter blood etc, and do a DNA profile that is virtually foolproof. Just like they do when a child's parentage is in doubt. You can take someone's fingerprints and compare them with those taken at a crime scene to see if you have a match. Hard evidence.
Bug stuff is a matter of opinion that can be "skewed" by many variables.