According to Bulbapedia and Serebii, as of Gen VI, Sacred Fire isn't anymore Ho-oh's Signature Move, because now Entei is able to learn it too. What do you think about it Share your opinion, I will share mine there as well.
I personally like it and hate it, I hate it because if Nintendo and Game Freak were doing it, they should have done something similar with Lugia's Aeroblast to keep the "Counterpartship". It isn't as "Sacred" now that someone else gets it.
But I like it because it's good for Entei as it isn't hurting itself anymore with Flare Blitz, given that the power difference is minimal.