Remember that opponents get to roll prior to the illusion to disregard it, but this is not always an issue.
RAW
Played as written, illusion spells are very hard to ignore. There are a few things that can make this a little better (for instance, a guy warning the rest of his group that something is an illusion), but that's not always going to happen or be feasible.
The main mechanical balancing factor for illusions is that they must be sustained. This is not necessarily horrible for a caster, but requires a quality or metamagic to be done without penalty.
It's also theoretically possible for foes to distrust something as an illusion. One thing I'd do as a GM is to make it so that if the illusion does not realistically respond the foes will ignore it; after a few high-caliber rounds hit the troll it "plays dead". It can fake being magically healed the next turn, but at least for a little while it's ignored. If it gets up from being obviously dead, I'd say the opponents get a bonus.
With SR5's tweaks to the skill system and skills going past 6, it's very easy to create an illusion-based character that is entirely impossible to counteract normally. This is because counteracting an illusion always requires the use of two mental attributes, which are notoriously difficult to increase past racial maximums.
Note that the rules for manipulation spells are not necessarily directly applicable; the way it works for those is that they are disrupted by real-world changes that unsettle the mage and make them "lose" the spell, rather than being any physical-side contravention of the magic.
Also note that Counterspelling can be used to protect everyone within the counterspeller's line of sight by giving them additional dice on their roll to resist magic. If the mage realizes an illusion is around, but can't notify others, they can also attempt to dispell.
4th Edition
In fourth Edition, illusions are resisted using either Willpower or Intuition but also with the Counterspelling skill, rather than with another mental attribute. In 4th Edition you couldn't usually get skills past 6 without a special quality or augmentations (which for Counterspelling was difficult), but at least it was possible.
3rd Edition
Illusions are resisted by Willpower or Intelligence [which was split into Intuition and Logic later]. If they're not resisted, they're in full effect.
General Overview
Illusions get one shot at being resisted, otherwise they take effect. There's nothing saying that people who are under their thrall can't realize they're fake, but they remain perceived until the mage stops sustaining the spell, or they are dispelled with counterspelling.
House Rules
I recommend the following potential solutions to illusions:
- Implement a "disbelief" counter; every time the illusion is physically interacted with and does not respond accordingly (including reacting to the effects of physical trauma), foes get to attempt their disbelief again, and if it reaches a rating equal to the hits on the spell everyone ignores it automatically.
- Resist with a skill, like Perception.
- Provide bonuses against illusions when dealing with known mages and when illusions have already been used recently.
