a large portion likely are indeed.
the qu'ran, hadeeth and all other holy books of all religions are written down years, decades, sometimes centuries after the events depicted in them happened.
telling stories father-to-son for that long will inevitably change them a bit: embellish them, your own morals and explanations will sneak in.
this is not to say that they are untrue, the core can be actual (although i personally don't believe in them, but that's a different story), any attempt to take any holy book (or written record of any kind) from that long ago literally is denying the truth: they *are* changed trough the years, simply by time, but also by translations (although for the qu'ran/hadeeth this is less of a factor).
Due to this: they all are unreliable, but not nessecarily untrue at the core