Orosei is useless card here. Why? Because creatures which cost 4,6,7 faeria have bad stats. In order to play Orosei you need creatures with cost 6. And transform them to 8 cost(The best stats in the game)
You’re totaly right on the number, but creatures with 3->5 cost are quite good (charger and manta). And with so many token, it is always valuable to transform. All games I used Orosei, she get me the win. Plus Orosei is a good way to heal your creature.
But you should see Orosei as a bonus card, not the main synergy. Istanu uses the same synergy which is good as well, and in the end, the colossus are your win condition most of the time.