Many people have proposed using biometrics to authenticate people to servers, but that's not what Apple is doing. Your fingerprint only authenticates you to your phone. So someone would have to copy your fingerprint and steal your phone to commit fraud. And if your phone is stolen, the fingerprint just needs to secure your phone long enough until you can remotely wipe it.
I'd really like to see how such an exploit would work before losing sleep over it. Fingerprint tech has moved a long way forward in the past few years. If it doesn't work, we don't need to use it.
So over the years of use I have to remember which ones have been compromised? Unlocking the iPhone with the ring finger of my off hand defeats the whole point of the convenience this is supposed to bring.