My 2 mth old slept in a playpen during the 1st month. From 2nd month, have been sleeping in yaolan in the afternoon , and on our bed at night. So far, no problem with his back or breathing.
Actually, i would prefer him not to sleep in yaolan as worried he'll get used to it, but weather nowadays is too hot. I don't want him to get used to sleeping in aircon, and he doesnt sleep too long on the playpen as he'll get sweaty back and will then start crying. For sarong, at least has ventilation holes and he feels cooler. I just rock him to sleep, and once he's asleep, we dun rock anymore.