For the most part, the “nasty” abilities of creatures and encounters are what pathfinder is all about - learning how to mitigate (or annihilate) the threats before they trash your party.
However, on the specific topic regarding the arrival of new waves of creatures, it’s actually being handled in a pretty unfair manner.
When a new creature arrives unstealthed, unless they have extremely high movement speed, and are charging out of the sky etc, they should:
- roll initiative
- arrive on their turn (burning actions)
- be able to be attacked immediately
- wait an entire round
- and only then get their turns.
The current implementation instead lets them:
- arrive (often right on top of characters)
- roll inititiative (hidden),
- and are generally un-attackable until they move (but since they may roll high on the initiative, they may get a turn right after arriving).
- Once the creatures move, they are placed into initiative,
but since they’ve gotten a “free” arrival move completely independent of initiative, this can feel a bit “gargoyle appears and eats the wizard” because they can fly down, then full attack immediately in some situations.
This is not correct per the rules, and should be fixed so that “arrival” provides players with a full round of reaction opportunity.
Stealth is handled a little differently already, but even then the full round of reactivity stops the majority of full attacks out of stealth (because they need to move to a target then stab) and also allows perception to function as intended, to catch them before they reach you.
But non-stealthed creatures should be arriving already in initiative order and thus burning actions/turns - they currently do not do this properly, and thus get “free” turns, which can feel unfair/wonky.
(There’s also some bugs with stealth where things are being revealed to players when the game is trying to hide them from you, such as the “see through geometry” window highlighting their location, their portrait showing up in the initiave order while they are taking stealthed turns, before disappearing again, and selective channel de-stealthing enemies - so there are some things that give freebies to players, but those are somewhat separate issues).