I love the Auto-fill feature on iOS and i also love buttercup but it has two issues: the app sometimes crash and the auto-fill is not like other password managers.
I don't know why the app crashes and i think this will be fixed by the programmers, but i wanna suggest to make people save a link in the password manager and when the device try to log in that link, buttercup will automarically fill username and password. I think with this feature buttercup will be perfect. If you wanna see in details how this will be you can try the "Avira PWM"'s auto-fill.