Hi Lucia,
yes taking on other peoples' energies is a problem for many, and especially for therapists. You're right that releasing these energies and emotions is crucial for maintaining our health.This is not specific to systemic work though, and the healing here is mainly meant to happen for the system that is acted out in the constellation.
When a foreign energy seems to stay, it is because it reflects a part of us we may have shut out - just like in the system, every part belongs there and that needs to be accepted before healing happens.

I do encourage everybody to practise energy discharge with the help of outbreath, of grounding, of movement, or neurogenic tremor as discussed in previous sessions