Rear facing offers the best protection of the cervical spine in a crash. That's the area at the back of the neck, just below the skull. This is true even of adults in the form of "whiplash". It's not a good thing for any person.
But young children don't have the strength in their neck to hold there head back. An adults head is approximately 6% of their body weight, whereas a young child's head (under 3 years old) is around 25% of the body weight. Their heads are about 4 times as heavy as an adults! A big difference. Kids are "bobbleheads".
Upon inpact, particularly front impact, a young child's head is thrown forward with tremendous force, and potentially can stress, stretch, or even break the spinal cord.
The advantages to front facing are mostly for the parent. The children are "easier" to put in the car seat, and the parents can see the childs face when driving. Nice, but not as safe.
And the other "reason" I've heard to front face a child is that it's more "comfortable" for their legs. Rearfacing, their legs are all "bent up", or may get broken in an accident. My reply is: I'd rather deal with a broken leg than a broken neck, paralysis, or ....death! And as for their legs being cramped up, all kids sit with their legs in awkward positions. Watch them on the couch, or the floor, or in a chair. Their legs are like rubber. They'll adapt.
There is a powerful movement toward extended rear facing children. One major obstacle is that car seat manufacturers currently don't offer RF car seats past 35lbs in the USA. They do in many European countries such as Sweden, Denmark and Finland. RF car seats as high as 55lbs!
On the next page there are several links that will give you the facts on rear facing.
Spend a little time and read them!!.
They may save you, or a friend, from a horrific experience someday