There is no challenge to be had in comparing Stealth and passive Perception and telling the players how much damage they take. That is randomness, not challenge because the players have almost no control over the outcome except how well they pumped passive Perception.
And the players may not be aware of it at the end of the day, despite the telegraphing. They may still get jumped by those gargoyles or piercers. That the DM telegraphed the threat, however, means that the chances of it being perceived as a "gotcha" is greatly reduced. (It depends on how artfully the DM telegraphed, really.)
It's a mistake to think that players always pick up on the clues, make the right decisions, and remove the challenge. At worst, they reduce the difficulty... but that's okay! It's a reward for smart play.
My players do not always pick up on my clues either.
But, there is no way I am going to telegraph an ambush predator.
Other predators? Sloppy ones that leave carcasses on the ground? Sure. The carcasses on the ground are the additional clue that there might be some type of monster in the vicinity.
But I disagree with your assessment that it is pure randomness when clues are not telegraphed. My players fan out when they enter a room. My players keep a lookout. My players have the best trap finders head towards interesting objects. This is not randomness, even though there might be an ambush predator in the room. This is purposeful decision making on the part of the players based on the information that they have received so far. They see a statue? That's an interesting object because most rooms do not have statues. They might go check it out, but they won't just all surround it and potentially get surprised with an area effect. They'll make (usually) intelligent decisions based on known information at the time.
In fact, I don't even give stealth vs. perception in the case of a gargoyle. The PCs can see the statue and I will describe things like wings, claws, fangs, etc. There is not necessarily an immediate perception to notice that something is not quite right. There might be a perception to notice that the statue has amazing detail (as per false appearance), but to determine that, they typically have to look a bit more closely at the statue and/or examine it in detail, and not just see it from across the room. No freebies. If they want more information, they have to interact with the gaming world, and not just roll dice or have the DM telegraph info.
The PCs in my group also get to use group stealth checks. So although the monsters sometimes ambush them, they sometimes ambush the monsters as well. Monsters in my world don't get info telegraphed from the DM either. Some times, the PCs get in a fight and the monsters (especially those with weapons, armor, and shields) are not ready for them. Oh well. Monsters can be sleeping, eating, playing, whatever based on monster, and not just waiting for the PCs to open the door so that they can fight right away.