Ho-Oh's power comes from something that is, I believe, more frightening than Metagross' CB Meteor Mash or Garchomp's Earthquake. Sacred Fire, as a move, and an effect, is something that very few Pokemon can switch into (though I don't like the whole *must be able to switch in to count as a counter* thing). Physical walls, Skarmory and Rhyperior, don't like the chance of a burn (and Skarmory doesn't like the super-effective hit). Guts users generally don't like the fire-typing. Non-water Pokemon don't like the chance of burn, either, so unless they're carrying a Lum Berry, they're not going to switch in. Flash Fire Pokemon have the chance of getting hit with Earthquake, especially Heatran. And then, there's always the threat of mixed sets (I really like the Sacred Fire/Thunderbolt moveset I attacked earlier).