you'd want the nut functions in-client so you don't need to log into a website. Just by creating an npub you're good to go. Asking people to download wallets is an extra step.
Yes, on ios users need to redeem from the npub.cash website, however...
nostr:npub1kvaln6tm0re4d99q9e4ma788wpvnw0jzkz595cljtfgwhldd75xsj9tkzv Wallet has a LN to ecash bridge built in. When a note is zapped it automatically arrives as ecash in the Minibits wallet when you put a minibits.cash address in your nostr profile for android.
npub.cash requires extra steps in redeeming from the website which you have to log into first then use nostr:npub1dx5q2el8nd4eh3eg9t2e25fd7zuqg7zxz6ldkc3uzgh66ss2yc6st288sj wallet on ios
Discussion
My point is that Minibits has already simplified ecash and nostr greatly and that it is currently the most minimalistic custodial setup.