There has been an extremely sad case where a little girl has died after being left alone in a bednest co-sleeper cot.
The story is here.
The bednest is not inherently dangerous, the problem occurs when the setting with the very low barrier between a parent's bed and the cot is used and the baby is placed on the front to sleep.
It's really upset me as I would imagine this would be the option chosen by parents who want the benefits of co sleeping but didn't want to risk the baby rolling into the adult bed. Please, if you're doing this, stop and use either the "safe bridge" set up or put both sides up entirely. It does also seem as though the problem was caused in part by the baby being on their front, but in any case a baby can roll without warning - safer not to use it at all.
The poor family :(