The thing is for any collision there are normally 2 cars involved. So for any rear end shunt it's still a frontal impact for the car behind. Same for a side impact if you're T-boned (less so if you're side swiped e.g. on motorway).
But anyway, regardless of which crash types are most common, if you look at everything:
Frontal collisions - RF vastly safer due to holding the head, spine and neck in alignment.
Side impacts - RF actually performs slightly better in real world due to pre-crash braking, which pushes a FF child out of the seat and outside but in a lab where the side impact test is a bench with the car seat on it and a solid metal panel coming towards it, RF/FF makes no difference. You find old sites saying RF seats have less side impact protection than FF seats - that was true about 10/15 years ago when SIP was a new thing. It's not really true now, most companies making RF seats have caught up. Older design RF seats still have poor SIP, but so do cheaper FF seats.
Rear impacts - RF very slightly safer still - as even in a rear shunt you're being pushed towards the front of the car, not the back of it. Not as big a difference as a frontal impact, I believe.
Reverse impact (e.g. reversing into another car/post/barrier) - FF is safer, but as these accidents tend to be at very low speeds it doesn't really matter.
Rollovers - RF/FF doesn't matter but you want your DC in a harness and not in an impact shield.