You are definitely correct about the more extreme mechanical movements of the vertibrae being the one thing that any back protectector can not protect one from. Even the hard segmented kind that limit hyperextension can only take minimal force before giving way. So landing on an object large enough to displace your spinal segments beyond normal limits is something you can't do anything about. Smaller stuff, then yes, you can protect soft tissue and even prevent the pointy bits of veribrae from breaking/crushing/cracking by reducing the Gs transmitted to it and preventing very shallow mechanical extensions.
But I am a believer in them for less extreme mechanical movements and I actually put one to the test this past September. I had a Rukka SRO Anatomic jacket that had a thick back protector built in that is a 3D lattice-like construction that stiffens on impact and spreads load. It looks similar to the lattice work on the Forcefield one in the picture for this article and is 4 lattice layers thick. (Rukka APS armour)
Well, my Guzzi NTX broke a lower fork casting that clamps the axle, locked the front wheel and sent me skyward at 80 mph on the TransLabrador Hwy in Labrador. It's rough and rocky over every sqaure meter or road for 1000 miles. I did my absolute best to land in a shoulder roll so that I couold lay out flat into a side fall like I would in martial arts from a throw. However my shoulder took most of the Gs from such a hig fall but i did roll onto my back, flattened out until I slid to a stop. I was lying on top of all kinds on rocks, generally no larger than a golf ball but a few a large as a cricket ball. The back protector did an amazing job as I had no bruising on my back nor broken pieces of spine. I was sore as fuck hours later though.
So, to me, the back protector did its job and did it well. Enough to make me believe that the outcome could have been much worse without it considering my clavicle ended up in 5 pieces with 2 cracked ribs. Also not related to the back protector were my Scorpion EXO-900 helmet taking a large hit during trying to position myself for a roll and slide (seems I got my head on the ground first, ok, I'm not a cat). It K.O.'d me. No idea how long I was out. So, the fact that the rest of my body turned out as well as it did at that speed and a fall from so high, I bought another Rukka with that bad ass armour as the SRO I was wearing was no longer 100% fit for duty. It was worth it.
On the street where things are flatter (excepting the worst of the potholed roads) I'd say these back protectors will do even better. Just mind the curbs.