It can take a hit the first time, but after second time around it’s more shaky and especially with hazards up.Zamazenta-H can still take a hit and absolutely shut down CB sets as long as you don't get flinched too much which is massive, also lots of pokemon are getting scald back, which should also keep it in line with all the burns being thrown around, especially with pex/glowking regaining scald, heatran, and volcanion around.
Do you know what a sidegrade means?Chien-Pao isn't a sidegrade to Zacian-H at all. Chien-Pao is weak to rocks, as poor bulk (80/80/65) and bad defensive typing. Pao is weak to rock, fire, fighting, steel, bug and fairy. All of this makes it harder for Pao to switch in repeatedly or get its boosts and sweep.
Zacian has very good bulk for a purely offensive Mon (92/115/115), and is weak to steel and poison only, two uncommon offensive typing. It's also not weak to hazards or U-turn. This gives Zacian plenty of opportunity to setup since there are very few OU mons that can force it out. Zacian is also less reliant on tera, so if Tera gets banned later, not much would change for it.
Chien-Pao has better ability, coverage, priority, and has dual STAB, but trades it off for significant survivability that post nerf-Zacian-H has and its speed (which outspeeds 2 relevant speed tiers. +1 Adamant Bax and Chien-Pao itself).
And reminder that I’m not actually saying Zacian should be unbanned, just that Chien-Pao should.