For instant payments you are trusting that the Ark operator and the sender of the payment won't collude to double spend it. If your balance in the form of VTXOs (virtual UTXOs, valid Bitcoin txns that create outputs onchain but kept unbroadcast) has made it to the onchain txn containing the tree structure that the Ark provider makes periodically, then you have unilateral exit and don't have any trust. One issue is that to unilaterally exit you will have to publish multiple onchain txns which makes it not feasible to exit if your balance is too low
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