FrostHeart Phoenix dont have Frostbite.
Its already a meme that Frostheart Phoenix doesnt slow anything, even though its a giant frost creature.
Chilling aura is overall better and really strong on a flying creature because you dont need to stay engaged to apply it, so it may be slightly overdo.
Frostbyte will gave FrostHeart phoenix atleast some role on a battlefield. It will slow enemy cavalry or monster for your cavalry or archers. It will finally slow stuff!
It is tankier than a flamespyre phoenix or even a sun dragon, but not acrane Phoenix. But it cant score any hits with 35 MA on enemy valuable targets, (you need 40+ for it) And it doesnt stop enemy units from reaching your other, less tanky units - because it doesnt slow.
Without AOE damage that both other phoenixes have, or all dragons have he left alone in a shadows... forsaken and cold...
Frostheart never was competitive, and i am sure that with frostbyte he will finally become one.