Privacy: NIP-60 proposes a new event kind so it would be visible that there's activity. The ecash coins themselves are encrypted but the number of coins (which is visible) *might* allow correlations (realistically speaking, I'm not concerned by this). Example: "Martti seems to have 10 encrypted coins on this relay... I'm guessing that's around 10000 sats but who knows, could be 10, could be a million".

It might make sense to use a different key pair for NIP-60 that's derived from your OG keys, but that would be harder to agree on.

NIP-61 is about using these ecash coins for zaps so the whole idea there is to be public.