Sleeping with a baby on your chest is really never okay.
The reason that you do not want to sleep with a baby on your chest is because it's possible that while you're asleep and the baby is on your chest you could roll over and crush or smother the baby.
There have been cases where some babies younger than a year old have been smothered and crushed by their mother or father while they had the baby sleeping on their chest.
If you're sleeping on a couch then there's the potential of dropping the baby as well.
As long as you're awake it's okay to allow the baby to sleep on your chest in a chair or on the couch.
But when you're sleeping the baby should be asleep in a crib or bassinet so that you do not crush or smother the baby.