As it stands right now, the Pelagornis is basically a worse Pteranadon that can land on water. I think that the creature could have a pretty unique use if the following changes were added.
1. Allow weapons to be used while mounting - would allow you to solo tame Quetzals and other Pelagornis easier (and I guess Megalodons?)
2. Allow you to carry small creatures with the beak like the Argentavis (dodos, dilos and the like)
3. Buff weight. The weight is pretty weaksauce, even at higher levels.