Great Question!
It is safe, as long as it is allowed by the carseat owner's manual.
First, in any vehicle in any crash, the ride down time is almost completely provided by the crushing frame of the vehicle. Some may also be provided by airbags or special seatbelt features.
Child restraints themselves provide little ride-down time of their own. The objective is to couple them solidly to the vehicle to take advantage of the crushing frame. If a child seat or harness is loose, the child will be flung into the harness at the same speed as the vehicles in the crash. If the loose install doesn't cause the child to strike something else in the vehicle, it will cause a sudden jolt to the child's internal organs when the harness does suddenly stop their movement. With a very loose install, you loose the benefit of the vehicle's crushing frame on ride-down time almost completely.
In the case of a rear-facing carseat, kids are well cradled in serious frontal crashes by the shell of the carseat. What we want to minimize is the motion of the child's head toward the front of the vehicle. The top of a rear-facing carseat will tend to rotate downward in a crash. If the child slips toward the front of the vehicle beyond the limit of the shell of the carseat, the top of their head could strike a vehicle seat or other object. If they slip way out of the confines of the shell, their heads may bend backward. That will be a risk of whiplash-type injuries as well.
To minimize this risk, you can do a few things. First, less recline is actually safer for kids above 6 months or so, with adequate neck strength and head support. Second, make sure the harness is tight and at or below their shoulders. Finally, bracing against the seat in front will also reduce this downward rotation. Alternately, the same thing can be accomplished with a Britax rear-facing tether in the "Australian" or TRV (Toward the Rear of the Vehicle) method.
Another risk gaining interest is the issue of what happens on the rebound of a rear-facing carseat backward into a vehicle seat. New studies may show that various minor facial injuries can result. While rear-facing carseats are still significantly safer than front-facing carseats, even these injuries can be prevented. Some seats offer a rear-facing tether that can be used "Swedish" style or TFV (Toward the Front of the Vehicle). These will prevent rebound into the vehicle seat. A "rebound bar" found on a few models will accomplish the same thing. TFV tethering can also help get the necessary recline for newborns and small infants. It may also provide more stability and safety in side impacts and rollovers.
I hope this wasn't too confusing
If you have any followup questions or want more clarification, please ask!
Darren