I'm no expert, but from the little I have studied it, and from what I know about the insect world, I think it is an underuitlized and potentially VERY useful tool in forensics. I think it is every bit as useful as DNA evidence.
Just as in DNA evidence, the handling of the sample is crucial. A wrong identification of the exact species could really screw you up, for example. The body definitely IS affected by variables in the environment, but the variables that are important can be fairly reliably verified, I think. For instance, temperatures and RH are always taken into account in forensic entomology. I believe temperature is the most critical variable, by far. Of course, microclimates are very important. For instance, if a body is found in a barn, house, or greenhouse, temperatures from the local airport may not help you build a very accurate picture. If a body is found in the shade in the summer, the degree days can be adjusted by taking the temperatures at the point where the body was found, and comparing them to the local temperatures that are on record, wherever those are gathered.
BTW, forensic entomology, as you may know, more often than not does not involve murder cases. More common cases are civil suits involving wood-destroying organisms or pests of stored products.