You get plenty of tries to perfect the technique, before using it on the actual device.
You acquire identical hardware and "dead finger countermeasures" (does the iphone employ any? Some readers look for pulses and whatnot, I don't know if the iphone does). You then practice reading the fingerprint on that hardware until you are able to reliably get a clean print and bypass any countermeasures. Only then do you try using the finger on the target phone.
You might still fuck it up, and you only get 5 chances on the target hardware. But with practice on the right hardware, I see no reason why you couldn't get it.