Ho-oh doesn't destroy anything with a reflect up, and reflect + toxic is a death sentence for him. I also fail to see how Espeon (the other bouncer) would stop a switch to Ho-oh. Sure Espeon could run the same set as Xatu, but he has worse defenses and doesn't have roost to back it up.
Espeon's overall bulk is actually slightly superior to Xatu's (65/65/95 for Espeon vs. 65/70/70 for Xatu). Xatu can switch in much easier on defensive Groudon, which is great news, but if an offensive Groudon uses Stone Edge or Rock Slide you will lose. Also note that Espeon has access to Morning Sun, which is actually better than Roost in the sun. I do agree that Ho-Oh cannot switch in safely on Xatu as Ho-Oh hates both Toxic and Thunder Wave.
I get that, but like I said above I learned really quickly to NOT put hazards up when I see a team with Espeon, and instead just throw off an attack when I predict a switch. If you don't bounce anything, all Espeon does is delay the set up of hazards and maybe hit a slower target for half health if the other guy stays in, but you're essentially playing 5 on 6 to get that. Xatu can switch in on earthquakes with its typing or weaker attacks like Deoxys-s fire punch and get up screens and/or toxic, then roost off the damage.
That is all Espeon needs to do to do its job. Entry hazards don't need to be held off the entire match; just for a good chunk of it. Espeon also has the advantage of actually having a usable special attack stat and decent movepool, though Xatu's flying typing and access to Thunder Wave makes it perfectly viable as well.