+ there are > 50 cashu mints I don't think that's a good explanation.

I'd agree at least partly about Ark. From what I remember anyway. Not knowledgeable enough about BitVM current ideas, but obviously not having centralized control of the money being exchanged is going to affect architecture etc. Lots of gray areas right, like Eth L2s being centralized when you look under the hood.

Oh but this is not a counter to my point imo. The basic architecture of a hub and spoke interaction with a central server is what I mean. Otoh if development requires interoperability w/ Lightning clearly that bumps the difficulty.

Can you say more about Ark? Assuming we have a large number of ASPs, then it's not centralised right? Or maybe Ark will naturally gravitate towards a small number of very-large ASPs? Although, someone could argue that Cashu might tend towards one large mint that everyone uses

I see all these systems with a client server relationship as at least quasi-centralized. Including nostr, though that's much lower on the sliding scale of "how problematically centralized is this".