sacobred What do you mean with "secure" here? Against what threat? You have to authorize each payment with pin and/or biometrics. The app is isolated just like any other GrapheneOS app. Curve itself is a 10-year-old fintech company with 5+ million clients in Europe, and they are subject to the EU laws for financial institutions. Payments go through your credit card ultimately, so you can always rely on your bank to challenge one.
(To avoid confusion: there is another similarly-named crypto company named "Curve Finance", but the two companies have nothing to do with each other.)