door bouncer safety

[MotherofHobbit]

We've bought our 4month old DS a door bouncer and he loves it!
My mother said she thought they could be bad for the baby's back or spine. This sounds vaguely familiar to me too.
The only info I can find on dangers relates to not installing them properly and the baby falling or hitting the door.
Does anyone know if there is any actual research or evidence for my spine/back damage theory or if we're imagining we heard this?

When you said 4 months, I did think that seems very young. I would have thought it would be once they are sitting up completely unaided.

They're supposed to be suitable from 3 months - as soon as they can hold their heads independently. My little giant is 20lbs though and very strong.

I thought and not sure where I got this from, that they should be in it for 20 mins max each session?