Great question.

Essentially, eCash on Cashu is custodial lightning with some extra steps that allow it to be more private. The mint can't see your balance or who is sending sats out of the mint. It's free-banking on lightning.

It is NOT self-sovereign. You are trusting the mint with your money, just like any other custodial relationship.

The mint goes away and your ability to exit back to lightning goes away with it. They can rug you at any time.