The Disadvantages: There are many advantages to using a plastic septic tank over a concrete septic tank, which includes, but are not limited to:

Plastic septic tanks tend to have lower effluent levels, which may be discovered after a tank is opened for pumping.
If not installed properly, a plastic septic tank can "float" to the surface of the ground.
Because plastic is such a light-weight material, it can be damaged at installation.
Plastic septic tanks are not approved in all states. (Find resources for your state here.)

When is a plastic septic tank appropriate? Plastic septic tanks can be the right choice when they are permissible in your state and when the cost is a significant consideration. In the end, you must decide if a plastic septic tank is best for your project