I realized that I said reparenting could be risky but I never said what I thought the risk was.

The risk has to do with transference and enactments. Plenty of people get those anyway, but when the T is deliberately stepping into the role of parent, it can increase the intensity of the transferential situation. This can bring out a lot of things such as very strong dependent feelings, trauma memories, anger/rage, power struggles, etc. just depending on the individual. Then if the T doesn't know how to handle those, it could cause the therapy to fall apart in possibly dramatic ways. That is a risk of any kind of therapy where there is a close supportive relationship but you might hope with someone who is doing the reparenting deliberately, that they might have more of a clue how to handle what comes up. Still, this clearly isn't always the case...