Hello not a mom, just joined to add my thoughts.
I purchased a 2004 volvo v70 wagon last year purposely for the third row seat. We both hate minivans and suv's just use to much gas.
Before getting the car, I tried researching the safety of these seats as much as possible. I was really concerned with the fact that the seats were rear facing. There is not much testing in the industry regarding these seats or any third row seats since the incidence of a rear-facing/rear-ending fatal collision is very low. However, many car companies are adding stow-away 3rd row seats. I have sat and looked at many of these seats, the head rest and back of the seat are very close to the rear windshield/back of the car! Not only that, there is usually little to no leg room between the 2nd row and third row seats. In the unfortunate event of an accident involving the rear of the car, I feel safer knowing my child is closer to the c-pillar(still an integral part of the car's "safety cage") than the very rear of the car!
If you follow Volvo's safety parameters(50-80 lbs, can't remember height restrictions), there should not be any issues. These seats as well as the 3-point seatbelts are anchored to the car's frame. I would just be cautious about leaving any objects untethered in the rear while a child is sitting there.
My son, who's almost 5, loves it in the back on the rare occasion we do need to use the third row. The only thing is that I'm thinking of tinting the rear windshield so that headlights at night do not blind him, although he insists that they do not.
Hope this helps.