Plastic bullets as designed for subduing or controlling people come in two main varieties: baton rounds and rubber bullets.
Plastic baton rounds are not defined as a non-lethal weapon, merely a less-lethal weapon. They are designed primarily to stun and knock people down. However the effect of the weapon is not predictable and these projectiles may have lethal effects.
What is generally known as rubber bullets are bullets designed to be fired at the ground and bounce in a suitably low trajectory in front of crowds to disperse them and cause only superficial wounds to the lower part of people's bodies (unless of course people lie down or hunch, which is likely to happen if you shoot at them at demonstrations). They have been used in Northern Ireland in the past but were replaced by the plastic baton round because they were considered too dangerous after many fatalities, and more recently in Palestine by Israeli forces. Rubber bullets also come in two varieties: all-rubber bullets and the more insidious rubber-coated metal bullets, which is essentially just as lethal as an ordinary bullet if fired directly at a person.
Without a doubt, it is extremely uncomfortable getting your kneecaps crushed by a plastic bullet, almost as uncomfortable as getting it shattered by a real bullet.
Either way, being hit in the head by a plastic bullet, either indirectly by a ricochet or directly essentially causes the same effect as an ordinary bullet: You are likely to end up dead. | 
