I haven't looked into Ark yet closely. But my concern is potentially the same -- this question of "unilateral exit" is so much less important than "is this this decentralized?" Spark, as we've seen, is about as decentralized as PayPal. You have to use their web servers to do anything. What about Ark? How many Ark service providers are there, and how easy is it to spin up a new one? The great power of Lightning and Bitcoin is that there are THOUSANDS of nodes, of both kinds. That kind of decentralization brings enormous benefits -- privacy, and also fault-tolerance.