Like I said, the difference is one of degree. When you decide that a white dragon lives in the mountains near Neverwinter, you're creating your own world and making a ruling fitting the game to your group. When you decide that centaurs are a playable race in your game, you're also adding to your world an making a ruling fitting the game to your group. Turn that white dragon into a monster from
Fifth Edition Foes and turn that centaur into a dragonborn, and it's still the same kind of decision. When you decide that a 15 on 1d12+1d8 is an encounter with 2d4 owlbears, you're doing the same thing. When you declare that a high Perception check reveals the hidden doors in a room, it's still the same kind of thought process. The difference is mostly in how big a potential impact these decisions have on the game going forward - a centaur or dragonborn in the party is a larger-scale decision tha "this room has hidden doors," which itself by the same token is probably smaller in scale than using a third party monster in your dungeon.
Or to put it another way: your decision to let Perception search a room isn't any less significant than your decision to allow the Noble from EN5IDER.
I mean, I probably am, but not for these reasons.
To maybe clarify a bit: I don't think that having to make a ruling on Stealth is all that different from making a ruling on whether or not to allow a 3PP race, and so if a DM is comfortable doing one but uncomfortable doing the other (ie, you ban all 3PP content because you don't want to make that ruling), it undermines some of my confidence in that DM, and might make me question if I'd be comfortable in that DM's campaign. Not because of any specific thing that DM permitted or banned, but because that DM seems uncomfortable with making rulings to make the game better for their players, which is something I'd want my DMs to be comfortable with.