I read reports of people who had limbs amputated because of an overdose... death caused by opioids is due to respiratory depression and consequent hypoxia, so depending on the time and if it is resuscitated, there could be permanent damage.
The beautiful thing about Pentobarbital is that even though it causes death, it is protected from hypoxia, so it is without a doubt the best substance there is.
But for example, hydromorphone and midolazam have already been used as execution methods too, so an opioid + benzo combination should be super peaceful. For anesthesia in hospitals they use remifentanil + midolazam + ketamine, so they are effective and painless ways to go.