They recommend avoiding Waits because most information gets lost between them. Waits are also imprecise on BNET due to latency/syncing, but this imprecision is often pretty small.
For example, this DOESN'T work because of the Wait:
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BAD
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Storm Bolt
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Actions
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Wait 2.00 seconds
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Unit - Kill (Target unit of ability being cast)
Target unit of ability being cast no longer exists after the Wait.
This DOES work however, because there is no Wait:
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GOOD
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Storm Bolt
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Actions
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Unit - Kill (Target unit of ability being cast)
But I want to use a Wait, so how do we fix this issue? Well, we can use variables to keep track of the Target unit of ability being cast:
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Simple Non-MUI Fix
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Storm Bolt
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Actions
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Set VariableSet Target = (Target unit of ability being cast)
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Wait 2.00 seconds
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Unit - Kill Target
But this still isn't perfect.
If multiple units have the Storm Bolt ability then problems can occur, this is because they're both sharing the Target variable, and Target can only ever be set to one unit at a time.
This may cause situations in which a unit is never killed because it's no longer set as Target after the 2.00 second Wait. This is what we call a non-MUI trigger.
But that doesn't mean that this is necessarily bad. As long as only 1 Unit has the Storm Bolt ability, and as long as the cooldown is longer than the Wait, then it will work perfectly fine. This is because the trigger will always finish before you can cast the ability again, so Target won't ever get overwritten.
To make your trigger MUI you can use one of many options such as
Unit Indexing,
Hashtables, and
Dynamic Indexing to list a few. Unit Indexing is a personal favorite and is what I'd use to make this Invulnerability ability MUI.