Thinking aloud.

Unintended consequences is a fact of life. Even if the rat problem solvers did not scam the government, their solutions might still have unintended side effects.

Poison- It is going to be disagreeable to other living things eventually. AKA pollution

Predators - Some non rodent may face extinction. AKA invasive species.

Lastly, are we asking rat problem solvers to be smart enough to eliminate rats and yet be dumb or altruistic enough to eliminate their livelihood?