With Assault Vest, and not a single EV in SpD., Ho-Oh reaches 516 SpD. In top with a Base 106 HP, thats a fucking monstruosity.
The question is: "Why would you run Assault Vest Ho-Oh instead of Multiscale Lugia?" Well, the answer is: Offensive Pressure.
Every time Lugia switches into the battlefield, the worse thing you can spect from it is Toxic, Whilrwind, maybe Ice Beam or some type of passive damage. When Ho-Oh swtiches in, your next Pokémon will suffer a ridiculously powerful attack in form of Brave Bird or Sacred Fire. There aren't many Pokémon who enjoy taking those hits. Even with the lack of Life Orb, Ho-Oh has the power to cripple the opponent, and Sacred Fire is something that always matter, even if the Pokémon you are hitting resists it, there's still a 50% chance of becoming burned.
The lack of Roost is not a problem at all; Regenerator is a great ability -to say the least-, and makes things easier. BTW, C.Band sets doesn't run Roost and its performance is solid as a rock, due to this ability.
PD: Sry about my ingrish =P