"Kusei" refers to the smell that is harmful to health, such as the smell of carrion. This word is usually used to describe an unpleasant smell.
"Nioi" means a strong smell, which may be good or bad. This word can be used to describe all kinds of smells, not just those that are harmful to health.
For example, when you say "this hamburger stinks", it means that this hamburger tastes terrible and is harmful to health. And when you say "this garden smells of flowers", you mean that this garden smells of flowers, which may be good. Hope to adopt