It is possible for a soul retrieval to make things uncomfortable for a while. Nothing that would make the presenting symptoms worse, but, since it is in effect 'taking you back' to a time when the soul part left (the soul part's memories of that time are likely to be far more clear than yours), this can rake up uncomfortable memories. However, these are memories. The power of them to hurt you has gone. This passes in a few days, as the part reintegrates.
If the shamanic practitioner is not good, there is potential for you to be a lot worse off. This is not a healing crisis. It is incompetence.
Sadly, many shamanic practitioners do not believe in the enormous power that they are calling on. They will, therefore, advise you that you 'may feel worse before you feel better' in order to hedge their bets and avoid criticism. However, most clients don't expect any change either, so possibly no harm done!
This is an interesting discussion. I believe our physical reality is a reflection of our inner consciousness and if we believe our clients will have a crisis, then it is likely to show up, especially if we have told them to expect one - as a hypnotherapist I am well aware of the power of suggestion, even in the waking state.
Try asking "what must I believe about myself for my clients to react in this way?" and notice what answers bubble up - there may a limiting belief to release. Beliefs have a strong effect on reality in my view, but many are unconscious. If we can get them to the level of the conscious mind they are easily released and replaced with more empowering ones.