It's very dangerous to inject directly into the body such quantity of saline without medical skills.
Under medical monitoring, what is the interest of temporary results, when you can choose implants?
The difference is that it's not as permanent as implants. You can hide it easily on regular workdays, because it just won't be there.
Also, there's a huge price difference. Direct injections can cost as little as 5$ per 1000cc. Getting implants is just 1000x more expensive.
The body has no problem taking huge quantities of saline, because it's PH neutral. The body treats it as regular excess fluid that it will just absorb. You'll only be peeing a lot. The only medical danger is the injection spot. You might inject/pierce a blood vein or in the very worst case, cause infection.
But that risk is no greater than people who need to inject medicine into themselves daily, for example, people with Diabetes.
With the little exception that: Getting an infection when using diabetes medicine is a lot easier to explain to a doctor instead of 'pleasure saline'