I love uni, who doesn't? I've purchased and cleaned live urchins twice now and I find it difficult.
I run into one of two problems:
1) I am too aggressive while removing and cleaning the uni and it basically melts.
2) I am too gentle in cleaning and a bitter aftertaste remains on the surface of the uni.
One source I read implied that if the uni fall apart too easily then they might not be fresh enough? It seemed the uni were certainly alive when I bought them. Would letting them hang out, alive, in a clean batch of salt water help flush them as is true with some shellfish?