They'd be "safe" as long as the connecting piece of fabric wasn't pushed down far enough to touch where the harness straps come out of the shell - that's speaking in terms of them being highly unlikely to affect crash test performance.
That being said, they are still an after market product and as such unauthorized by any carseat manufacturer to be used specifically with their seat. I don't know why it would be anymore effective than using receiving blankets personally, other than not having to roll the blankets up.
If a parent really wanted to though, using the side pillows without the backing for a baby would be harmless since they'd be being used just like a receiving blanket.
It's kind of a fine line with something like this. Yes, the entire thing is an aftermarket product, however, using the pillows on either side of the head without the fabric behind the head would be no different than using receiving blankets, and as a tech I wouldn't have a problem with it. If someone came into a check with the back piece attached and behind the child, I would be inclined to detach the pillows and use them separately, but in the grand scheme of things and looking in terms of actual safety risk, these are pretty safe if used carefully.
eta: It's kind of a knock-off of the toddler pillow portion of the comfy soft - they just made the back detachable. Wonder if they'll actually be able to patent it... I rather doubt it.