It isn't similar man.

The full zanarkadian is a dream/summon, Tidus and his father only happen to be from there (perhaps even more previous guardians of the summoner), while there were some red hearings about Auron being a dead person.

Surprise!  Your party member is really a ghost.

AND

Surprise!  Your party member is really a dream.

They are pretty similar.  Your party member is fake somehow.  Then they fade out.

I get where you're coming from, since it's a similar effect used for both characters, but the meaning behind both characters vanishing are different. That being said, I've always said Auron being an unsent causes terrible issues to the plot, but I try not to think too much about it.

What are those issues out of interest? I think him being dead fits quite well in the whole thing.

I've always felt it creates a terrible conflict within the world's rules in itself. Aside from gameplay segregation (i.e. Auron dying and getting a game over, but these are game mechanics and thus should be looked at differently), I've never understood why should anyone fear death, nor why sending is that important. You're bound to be an unsent because you have unfinished business, and as long as you don't envy the living, you don't really become a fiend. Seymour basically learn this and keeps going over and over because he can't truly be killed unless forcibly sent, but it also highlights the fact that there's nothing stopping you from coming back from the Farplane - Seymour didn't even envy the living, being an unsent gave him a new outlook about life entirely.

Then there's Auron. Auron is an unsent because, well, he felt cheated by Yu Yevon and everything else, so he had to come back and make sure things stood right a second time. Why the hell does he age? How does Yuna can't felt an unsent presence so close to her? How come he didn't lose sanity? At what point do you actually have the willpower to come back from death and keep living your life? And if you can do this, why wouldn't some of the deceased do it as well? Auron is basically the prime example of posing the question why should anyone fear death in Spira? since he can basically camouflage himself as a living person and just happily stand with a bunch of guardians going on an adventure. It's explicitly stated (unless he's a damn great actor) that he feels emotions while being unsent (joking with Yuna at the temples or being extremely angry at Seymour), so there's literally zero downsides of his status. The only plausible thing I could think of is going insane and thus becoming a fiend - but since he's pretty much the same as a living person, any sane living person could go insane, get killed and come back as a powerful fiend, so the line that separates both is actually very, very thin.

There's people that die in peace, such as Tidus' mother. But not every person can die in peace, and not necessarily every one of these have to die envying the living. Especially when what's there to envy the living of, since you're pretty much a living person being unsent? Auron makes death/unsent rules in Spira look like a joke.

EDIT: I forgot to mention, Seymour's father despite being sent has the massive willpower to push himself out of the freaking Farplane to warn Yuna about Seymour. Seriously, nothing ever stays dead in this world, and has no reason to.

 or in the case of X-2, just being bought back for no reason.

Surprise!  Your party member is really a ghost.

AND

Surprise!  Your party member is really a dream.

They are pretty similar.  Your party member is fake somehow.  Then they fade out.

The implications are different, however. Auron was dead all along, but under the Death rules of Spira, he could come back to finish what he couldn't end before. He's not unlike Seymour, who dies several times throughout the game, or certain other characters that come briefly from Death before vanishing. He doesn't enter the Farplane because it would show (or he would be trapped due to) that he's dead. Tidus on the other hand can freely enter the Farplane because he isn't dead - he's merely not real.

The fact that they both vanish at the end is for two different reasons: Auron is an unsent - a deceased person, and therefore travels to the Farplane. Tidus is the recreation of a dream from the Fayth, who are also unsents, and because they aren't dreaming anymore, Tidus ceases to exist. There's a small moment of reunion between him and his father, surrounded by both Braska and Auron, but this still means Tidus will vanish entirely. Then there's the final sequence, with Tidus waking up elsewhere.

I get where you're coming from, since it's a similar effect used for both characters, but the meaning behind both characters vanishing are different. That being said, I've always said Auron being an unsent causes terrible issues to the plot, but I try not to think too much about it.

What are those issues out of interest? I think him being dead fits quite well in the whole thing.

On topic, the ending. The sacrifice Tidus made to save everyone on Spira was a great moment.

Just a shame it was all undone by a single scene and X-2.

I've always felt it creates a terrible conflict within the world's rules in itself. Aside from gameplay segregation (i.e. Auron dying and getting a game over, but these are game mechanics and thus should be looked at differently), I've never understood why should anyone fear death, nor why sending is that important. You're bound to be an unsent because you have unfinished business, and as long as you don't envy the living, you don't really become a fiend. Seymour basically learn this and keeps going over and over because he can't truly be killed unless forcibly sent, but it also highlights the fact that there's nothing stopping you from coming back from the Farplane - Seymour didn't even envy the living, being an unsent gave him a new outlook about life entirely.

Then there's Auron. Auron is an unsent because, well, he felt cheated by Yu Yevon and everything else, so he had to come back and make sure things stood right a second time. Why the hell does he age? How does Yuna can't felt an unsent presence so close to her? How come he didn't lose sanity? At what point do you actually have the willpower to come back from death and keep living your life? And if you can do this, why wouldn't some of the deceased do it as well? Auron is basically the prime example of posing the question why should anyone fear death in Spira? since he can basically camouflage himself as a living person and just happily stand with a bunch of guardians going on an adventure. It's explicitly stated (unless he's a damn great actor) that he feels emotions while being unsent (joking with Yuna at the temples or being extremely angry at Seymour), so there's literally zero downsides of his status. The only plausible thing I could think of is going insane and thus becoming a fiend - but since he's pretty much the same as a living person, any sane living person could go insane, get killed and come back as a powerful fiend, so the line that separates both is actually very, very thin.

There's people that die in peace, such as Tidus' mother. But not every person can die in peace, and not necessarily every one of these have to die envying the living. Especially when what's there to envy the living of, since you're pretty much a living person being unsent? Auron makes death/unsent rules in Spira look like a joke.

EDIT: I forgot to mention, Seymour's father despite being sent has the massive willpower to push himself out of the freaking Farplane to warn Yuna about Seymour. Seriously, nothing ever stays dead in this world, and has no reason to.