Im wondering if the reason he is designated that way, is the possibility he cannot die? At least in the traditional sense.
We know he can see, talk, and interact with ghosts. That alone confirms souls are very much a real thing, not a concept. So what if his body can die, but not his soul?
Therefore they believe he is a calamity because even if he were to die, he can freely use his powers even in a soul state (theoretically). His ability is also "hacking" so they assume (we already know and have seen) he can take over other people's bodies.
In conclusion, even if he dies, he isn't going anywhere but could come back in anyone else's body and can do it on repeat. Therefore he is unkillable and deserving of S-Class