Mostly logistic spells and abilities that may trivialize certain Scenarios. Teleport, fly, Locate x, misty step and such. But--- here comes the big but --- it really depends on the kind of campaign i want to run.
Have a smart Wizard do the same thing to them: On the far side of a cave, Illusion up some boulders to hide behind, and snipe at Team PC from there. See if the PCs shoot the boulders "to find out if they are real."
Follow this up with an OOC discussion of how 'common sense' will be changed in a magical world - that, you know, includes Illusions. Let them know that you will be making a Ruling at the beginning of next session, so you can consider the arguments made in all directions.
I thought this spell was now useless given that RAW says that players get a free identify with a short rest. I have house-ruled against that because I want my magic items to have a bit of mystery to them.
A problem I have with this group is that if I make the world act and react the same way they act and react to the world, they call BS. So they try to cheese illusions and enchantments, but if I do the same, I'm a bad DM out to kill their PC for fun.
Same if they treat my NPCs like crap. If my NPCs get angry or uncooperative after being rough up by the party, they are instantly branded as enemies to kill. For now we have a strict rule: if you intend to use Illusions or Enchantment spells that dont have a clear cut description of its effects, you have to tell me so I know ahead of time and can actually rule on it before you start using the spell at the table.
Anyway, its like most features: its not the features itself, its how you use it. If you use it to create a cool narrative to tell an interesting story for everybody at the table, its cool. Nobody will remember the time you ''cheesed'' suggestion to tell the orcs to stand with their foreheads to the wall doing nothing while you looted their cave for the third time.
This is a DM/group issue. I've always let my players know that turnabout is fair play. While I try to take mental capacity of the opponent into account (an ogre may be fooled by something most creatures would not), in most cases what's good for the goose is good for the gander. That includes the reputation and tactics the PCs use becoming well known just like the PCs know not to look at a Medusa.
As an example, I run a fairly high magic world, so if the NPCs are familiar with the area, they're going to know that if a wall magically appear in the middle of their house it's probably an illusion. If arrows start flying out of a rock, they're going to realize the rock is an illusion, no check required.
It sounds like you've dealt with it. In my experience some players are always going to complain about my running NPCs as real flesh and blood individuals, or complain when they don't completely dominate every encounter. It's been that way in every RPG I've ever played. I try to have a mix of encounters, and sometimes they stomp all over my carefully laid plans which is fine. But if they want to play D&D on "easy" mode I'm probably not the DM for them.
I hate identify because it reveals curses and inconvenient side-effects, totally removing the incentive to experiment with an item or risk using it without knowing exactly what it does. Plus, there's no cost or risk associated with identify since it's a ritual. It basically cancels out the official variant, bringing us back to "free identify with a short rest provided there's a wizard in your party."
Before 5th edition made identifying a potion only take having a sip of it, I saw more than one party die with unidentified healing potions on hand because the players thought the risk of drinking an unidentified potion was greater than the risk of running into more monsters while their characters had few hit points left.In every group I've ever played in, no one will use an item until it's identified, even back when this was a significant investment of time and resources. Better to let an item sit in your pack unidentified and unused than risk wielding a sword that will chat with you incessantly that you can't put down or a belt that will turn you into a girl that you can't take off.
I hate identify because it reveals curses and inconvenient side-effects