I can only talk about my own experiences as a past life regression therapist and a student of spiritual hypnotherapy and PLRT teachers, and a reader of New Age books and channelings.
It seems to me that ‘karma as punishment” is overrated, very rarely we get cases when client’s suffering in this life is a punishment for something very bad they did in a past life. Dolores Cannon has a case of a blind woman in one of her books, who saw a past life in regression when she was a cruel king, blinding people for disobedience, so in this life she was born blind as karmic retribution for her cruelty in the past life. There was a case in Michael Newton’s books of client who lost a child to an illness, and this was karmically connected with a past life when she was a wealthy woman in London in 19th century, laughing at poor women with children begging for money and food, and not giving them anything. I personally never had any clients who suffered in this life for a past life bad deeds, and I had clients who were Nazis in past lives, even a Nazi doctor doing horrible experiments on gay men in concentration camps…in this life she is a straight woman who is involved in gay rights activism. It seems that souls can counteract negative karma by doing good deeds, charity, volunteer work in this life - instead of suffering the karmic punishment.
More often we have cases of carryovers of illnesses, traumatic situations, depression from past lives into current life. Very typical are stories of past life suicides - they all are depressed in this life and have many painful situations similar to ones in the past life: loss of loved ones, miscarriages, divorces, betrayals, business ruin etc. They are put in similar situations to learn how to overcome grief or difficulty without “bailing out”, and PLRT sessions are very useful for them to overcome their suicidal tendencies. Traumatic situation repeats in each life till we learn the lesson and are able to heal ourselves.
Sometimes illness in this life can be chosen by a soul before birth for a reason, which has nothing to do with past lives karma, but to learn to heal themselves and others. These are usually “Healer Souls” who do spiritual or herbal healing or practice medicine in incarnations and in spiritual reality, as Spirit Healers/Medical Guides, and they often incarnate with a certain illness for the sake of experience, to better understand it and learn how to heal it, this is a “wounded healer” pattern. Similar pattern can also apply to mental illness, many psychiatrists or psychologists had past lives as mentally ill people, which gives them deeper insight on the suffering of their clients in this life.
We can also get born with disability or illness to provide lessons for other souls who need experience of being caregivers, to learn unconditional love and altruism.
When we connect with clients’ Spirit Guides and Higher Self in sessions - we often hear that most of their past lives do not affect their current life, there is no karma to release, except for 1–2 recent past lives which need to be acknowledged and healed, sometimes there are no carryovers at all and clients are taken into possible futures or higher dimensions instead of the past lives. Also there are many Volunteers from other planets and higher dimensional worlds incarnated in the human bodies nowadays, who have no karma whatsoever, as they either had no past lives on Earth at all, or ‘mission” lives, when they came to help humanity, and they can not create any negative karma in these lives. Dolores Cannon talks about them in the book ‘Three Waves of Volunteers and the New Earth”