Are car seat inserts/body cuddlers safe?

Sarah
Are car seat inserts/body cuddlers like this one safe to use in a car seat with a newborn? I'm due in a couple weeks and my mom got me one and insists I'll need it when my baby is just born to hold him and his head in place, but I've heard and read that they are not safe in an accident. I've heard that a better recommendation is to use rolled up swaddle blankets on each side of his head. What do you all think? 

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