Not according to graphene documentation. From what I remember it's because Obtaniuam can't check the signature of the APK. There is app verifier (recommended by obtanium) but it dosnt work with many apps as they don't publish the signature. So you can't verify the APK is what its supposed to be.
FDroid builds the app and signs it. While this may intorduce risk of Fdrpid are doing something dodgey the many eyes and build logs should pick it up.