I have been using the yaolan since my baby is 2 mths old. Now she is 7 mths old and still taking her naps in it. I make sure her night sleep is on the cot (I transferred her back to the cot after rocking her to sleep when she's 2 mths old). even when we travel, she can nap without the yaolan cos I substitue it with the pacifier. right now, she needs the pacifier to sleep in the yaolan.
As for affecting the spine devt, dont rock yr baby too hard. Make sure the floor is protected with a mattress in case the yaolan gives way. Let yr baby sleep in the cot at night by transferring her/him from the yaolan after she/he is sound asleep