Possession is also the only skill out of the entire Sadhu kit that deals decent sustained damage since OOB auto attacks are extremely weak and are only used as a filler.
Possession does not go through bloodletting and is removable by dispeller. There are many aoe disables in this game that do more damage than possession, are ranged, hits more targets, are not removable by dispeller and have shorter cast time. Those aoe disables also come with a better kit than sadhu due to having better prior classes.
If you want examples: Cryo (all their skills basically), Cata (pretty much all their skills are knockbacks), Psychokino (PP, Gravity Pole, Raise), any mage class due to sleep, any warrior with restrain. Naturally, many of these can be removed with dispeller and blocked with blood letting, but possession is the same.
Also, pvp is not a 1v1 affair, possession is easy to interrupt even with safety zone and Sterea Trofh. Also nobody with a brain will ever get hit by out of body auto attacks. Even in 1v1, there are many ccs that can result in someone dying before it ends that are far easier to land.
Also your mission example is definitely an over exaggeration and purely situational. Although if your team is bad enough many classes: a good necromancer, wugushi, or fletcher can do the exact same thing. They also don’t have 100% bonus damage due to the bear being weak to holy. They can also solo missions much easier than a Sadhu ever will.
P.S. all skills are easy to land in pve, even meteor and rune caster skills.
P.P.S. Any class with cc can 100-0 with a single cc landing in 1v1. That’s literally why 1v1 is not balanced.