Did people have the same complaints about bad writing when Beric Dondarrion's revival set the precedent for returning from the dead very early on in the story telling?
Sandor Clegane is never once established as dead in the story telling, only ever assumed by the audience. There is no death scene and no scene or line of dialogue putting him as dead. There is no random Lannister in a tavern somewhere going "Did you hear they found the body o' The Hound out in the middle of nowhere? Fuckin' traitor deserved it if you ask me." Turns out the dude is a big fucker and tough to kill. Maybe I didn't get that line exactly right, but fact is Sandor is a badass. Another badass is Syrio Forel, who we never see die. If he were to pop up nobody would say he returned from the dead, we would say he didn't die. And what do we say to the God of death?
Benjen isn't ever established as dead either, only assumed. His fate is often questioned by the characters throughout the story and even used by Alister and the others in the betrayal of Jon Snow to lure him into the stabby trap. But we never see him die, we are only left to our own imagination and it turns out he is (kind of) alive. Benjen and Gendry can draw comparison too, since people can assume whatever they want about the fate of Gendry because we haven't seen him, but we have no idea if he is dead or alive and him popping up wouldn't mean he returned from the dead.
Regarding The Mountain, we see almost immediately that Qyburn is working on the mortally wounded, poisoned, dying body of Gregor Clegane. We had scenes (either simultaneously or previously, can't remember the timeline) establishing Qyburn as a disgraced Maester who was punished/kicked out for his experiments on humans. We got small scenes like Qyburn working on Jamie after his hand was severed and Qyburn comments about the necrotic flesh, as well as a scene where adventurer's bring Cersei a severed dwarf head that isn't Tyrion and Qyburn requests to keep it for his studies/experiements. Not to mention it isn't like bringing back Gregor is returning a major player to the fold. He is nothing more than a golem/humonculus that serves the purpose of giving Cersei some level of power when she shouldn't have any.
Point is that nothing is just pulled out of nowhere. Characters that are alive, that were either shown to be dead or thought to be dead, either had their revival set up in the story telling or they were never explicitly established as dead in the first place. For everyone else, we either see their corpses in the crypts (Joffrey, Tywin) or we see them burned (Khal Drogo) or we see their skull crushed (Oberyn). In Stannis' case we don't see the death, but dialogue between characters establishes his death and leaves nothing in question. I believe that was a stylistic choice by the creators of the show. Stannis' dishonorable acts (assassination of family, sacrifices of men and family) earned him a fate of dying as a nobody on a battlefield and then being forgotten, rather than being honored/buried/remembered as a king.
If we want to say Gregor was revived from the dead then okay, but it is well established in the story telling, just like Jon Snow's revival. Personally I like my story telling to be things being set up and paid off like this. It would be horrible if we never had the set up of Thoros reviving Beric and then Melisandre just goes "oh I can probably bring Jon Snow back to life, okay hes back imma disappear from the story completely now, kbye". And as others have said I like that the magic in this universe is mystical and mysterious, not formulaic. We don't know if the Red God is a real entity or not, but he could be and for all we know for someone to be able to be returned from the dead the God may need to decide that they are worthy. I don't think a Red Priest or Priestess could establish a church in every town in this world and charge a gold sum based on their experience level to revive fallen adventurers.
We're not going to see Stannis, or Joffrey, Or Tywin, or Oberyn, or Ned, or Robb, or Lord Commander Mormont, or Viserys, or King Robert, or Renley, or Jojen, or Ygritte or Khal Drogo just pop up one episode and go "Hey I'm alive, surprise!". Anyway, I think the writing on the show has its flaws and falls short in some areas (especially this latest season, as many people seem to agree), but I don't think this area is one of them.