Concentrates Concentrate Everything
That sentence is the whole category in five words, and it cuts both ways.
Done well, extraction isolates the trichome — the resin heads where the cannabinoids and terpenes actually live — and leaves the plant material behind. Done badly, or done to material that should not have been used, it concentrates whatever else was on the plant right alongside them. Pesticide residue that would have been a marginal number on flower becomes a serious one after a tenfold concentration step.
This is why the testing bar here is higher than for flower, and why we care rather a lot about the input material being grown without synthetic pesticides, herbicides or chemical fertilizers. On flower that is a preference. On concentrates it is the difference between a clean product and a concentrated problem.