I think "the possible" that the fayth did was bring back a somewhat distorted Tidus.
You can't mess up with the dead, or you could get not only yourself, but the entire balance of the universe by trying.
The fact that we die is directly linked by the fact that we are born: we are only able to reproduce because one day we will die.
If we were immortal, there would be no need to reproduce, or else there would be countless living beings since they can reproduce, but can't die.
By doing such a profane act as plucking someone's soul from death itself, or, in other words, bringing someone back from the Farplane, you're messing with the plane's balance: death is supposed to be irreversible, no matter how much the living wishes it would be. It demeans life: why would you live life to the fullest if you can live again? By bringing Tidus back to life, the fayth couldn't put his essence back together, because even they have flaws and couldn't see him as more than just a dream fabricated by them.
So he essentially was like a new computer assembled from scratch with an old hard drive: he has his memories, knows he loves Yuna, but still has the mindset of the guy who lived at Zanarkand and was star player of a blitzball team. And, of course, everybody who knew him and knows that he had died were like 'wtf how is that even possible', since he is no unsent, since he went to the Farplane, but was not like Jyscal who ventured out of it on his own accord, and even then everyone went wtf.
It's not that he's corrupting the other characters, but that he's corrupted himself but is not aware of it and everyone finds it strange that he managed to be brought back to life from the Farplane.
The thing about resurrecting him many times is REALLY demeaning. The fact that we live only once gives our life meaning, makes us want to live our life to the fullest because we know we are going to die someday and will never get another chance to live. To have him resurrect that many times invalidates this feeling, and Tidus himself as a character and a person, and this is really sad.
EDIT: Notice that the Farplane was already kind of unstable during the events of FFX-2, probably due to the uncountable number of fayth that was sent there at the end of FFX, perhaps Sin's presence there and Vegnagun absorbing its energy. Plucking Tidus out of it once might have intensified that instability and, by the time he was pulled out of it a SECOND time (stahp!), it kind of melted down and those bizarre beckoning events started happening. That's why you don't mess with the realm of the dead.