What is important to recognize with boric acid is that it is ONLY a stomach poison. It must be ingested by the insect to cause a toxic effect. It absolutely is not a contact insecticide and will have no effect on an insect pest if it only rests on the exoskeleton. Thus, the insect must have some means for ingesting the boric acid, and many pests do not - bed bugs are the prime example. With their sucking mouth they have not ability to ingest a dust, so using boric acid dusts within wall voids for bed bugs is not going to have any effect on them. 

Once ingested the actual mode of action of boric acid may still not be totally understood, but in some manner it disrupts the ability of the insect to digest its food, and the insect eventually starves to death. This takes some time to complete, so boric acid is definitely not a fast acting material. However, it is very effective on those pest insects that can ingest it and so far no resistance has been seen by any kinds of insects. Boric acid is also a mineral and thus an Inorganic chemical and it lasts virtually forever. For bait products it will last well beyond the point that the bait is no longer palatable to the roach, and for dust products it will last and be effective as long as it remains dry. It is non-repellent and not detected by the insect as it ingests the material. As a dust product it works on roaches because of their grooming habits, whereby they use their mouthparts to clean off their legs and antennae and if some of the dust is on those areas the roach will ingest it. 

The internet is full of good information as well as awful information, and boric acid suffers badly from dis-information on many websites. Anti-pesticide groups and "natural pest control groups" like to recommend boric acid as an "alternative to toxic pesticides", but of course boric acid IS a pesticide and IS toxic. Some websites state it kills by desiccation, similar to silica gel and diatomaceous earth dusts, and this is completely false. Some websites state is is "safe for use around children" and "non-toxic to people" and both of these are not only false, they are dangerous comments that will lead homeowners to misuse the materials and place them where children or pets could contact and eat them. 

Boric acid is a great active ingredient with some limitations and some misconceptions, but it is excellent on roaches.