This is, honestly, why I feel illusion spells should be removed from the game - and I love illusionists since I read about them in 2nd Edition. The problem, however, is that every illusion spell is very malleable, by design. Yet, I've only met one DM who was capable of working with an illusionist and not just nerfing them off the cuff because they wanted too. My own experiences generally indicate one of two "rationales" for DMs: 1) the NPCs are hard core rational actors who, in the middle of a fight, will sit down and work out the logic of the PCs ability to create something from nothing a big component of this is that "magical effects don't happen in the real world so if fire were to appear out of nothing, the NPCs would know it was illusion" which conveniently ignores that magic is part of this world, and fire appearing out of nothing is something that most NPC monster's will have seen their shamans do, the inverse logic is 2) the NPCs are all experts on spellcraft and thus know an illusion when they see one. This is why I generally don't play an illusionist or a spellcaster. I find most DMs want to know EXACTLY what a spell/action will do, and if something is poorly defined in terms of damage rolls or DCs, then the DM is going to take the least favorable to the PC interpretation. If this offends someone reading this, I'm sorry, but that's been my experience.
In general, this is what I would expect to see happen:
1. Illusionist casts their spell, and a unicorn appears. I'd have the player describe what they're doing: is the Unicorn just manifesting out of thin air or is it "coming" from behind a corner? If "manifesting" I'd give an arcana check at disadvantage to the NPCs to see if they put together that it might be an illusion with the DC dependent on the caster. I'd allow an NPC wizard of equal or higher level to the PC, with direct line of sight and ability to see material components, hear incantations, take a normal skill arcana check - though the PC wizard can attempt an opposed deception check to mask their actions.
2. The object now exists and the NPCs can begin to interact with it. The orc fires an arrow and rolls very high. They "know" that their shot was "perfect." I allow the PC wizard controlling the illusion to make a deception check to make it look like the illusion dodges the arrow. If they aren't paying close attention (i.e. threatened within 5' by an enemy) I would impose disadvantage. The DC is the attack roll. If the PC passes, no matter how good the shot, the NPC knows that they "missed." If the PC fails, I'd allow an illusion saving throw for the orc. This is because the NPC didn't PHYSICALLY interact with the illusion. The ARROW physically interacted with the illusion. Silent image is designed to fool sight, and sight is the only sense that the orc has used so far to interact with the illusion.
3. The other orcs, seeing this illusion and seeing the arrow fire and miss realize that they have a new threat, and act accordingly. When an orc decides to charge the illusion, the power of silent image is over as they will have physically interacted at that point. Per the description. Then the others can get a saving throw.
This is exactly why I always speak to a new DM about illusions BEFORE using them. I also bring up PRE-GAME or POST GAME that an opponent has to spend an action to disbelieve it. Turn it around on the players, who know all about magic, at what point would a PC spend an action to dis-believe the illusion when there are so many other possible explanations.
You are right that many DM's nerf illusions, too much IMHO. However I have gotten a great deal of mileage out of simple conversations pre and post game. I won't argue with a ruling during a game. Most DM's are trying their best to make a fun game, they are only human and doing things on the fly. They may not remember the specific rules for a specific spell off the top of their head. Having pre and post game conversations makes this a little better. Also when a DM gets a feel for a well played illusionist, it can help them in several ways.
They get ideas to use for their monsters. They learn the spell rules a bit better. It is really easy to miss that whole 'spend and action to disbelieve.' Also by telling the DM what you are TRYING to do can really help them adjudicate it, and often it will be in your favor. I think of illusions as really minor Wish spells. If you try to abuse a wish the DM will try to twist it and you will get the short end of the stick. If you police yourself and don't get greedy, you often get much better results. So if the DM hears your description and what you are actually trying to accomplish, it makes his job easier and most DM's will reward that.
Also because I DM too, I hate rule arguments during the game, so when I play I save that for after and its amazing what being polite and having a conversation do. DM's appreciate NOT being interrupted and argued with mid game, that also helps. In doing that with several DM's I have gotten away with more than I thought I could and fun was had by all. I think the key is finding a baseline with a DM and if that baseline is such that illusions would be effectively nerfed, then play something else. It will save you much grief.
In the meantime try explaining your GOAL with an illusion, not just the description. If gives the DM something to base his ruling on. Most DM's are just trying to have fun, make it easier for them and you get more than you would expect.
Tips for illusions: If it is going to interact with creatures in combat, use as others have said spectral foes. Make a ghost rise from a fallen person. Make environmental effects, you can mimic fog for example. Search for illusions on the forum and write down good ideas. Don't do the same thing every time, that gets annoying from the DM perspective and is boring. Think of interesting things you can do to control and modify the area. Many spells can be nearly duplicated with illusions. And with 5e there are no opposition schools, so you can still throw some other things around. Its nice to be an illusionist with magic missile and fireball and summoning spells. Mix them around.
If you are DMing, ASK your player what they are really trying to accomplish. Look at the intelligence of a monster, and their culture etc. to see if there is a good reason for them to fall for or suspect an illusion. As a DM use illusions against the players and see how they react and it will give you a good idea of what a intelligent thinking enemy would do when confronted with them.
Like in many things communication is important. If you can make your DM's job easier you will likely get more satisfaction in the end. That said there are definitely DM's I would not bother taking any illusion spells with, they just can't or won't be fair. In that case I would get upset and the DM would get upset which is not fun so why bother playing if it is not fun? So I'll play something else.