When does "Level of ability for a unit" work as a trigger condition?

[Verdun]

In my triggers, I often rely on "level of ability for unit equal to 1" to check if a unit has a specific ability and can thus be used in the trigger, like only killing units who got shadow meld.

But every so often I discover limits to this approach.

Apparently, an ability being researched or not changes precisely nothing, at least for certain abilities?

The ability icon can be dark all it wants, the unit is still counted as having at least 1 level of the ability, and will be used by the trigger.
The only thing that reliably works is to disable the ability for the player altogether.

Is that a known issue?
Is that limited to some abilities, or a universal thing?

 

[Nichilus]

If unit has the ability, then ability level is always greater than 0, irrespective of whether that ability can be used or is disabled/researched.
Basically any ability you add in object editor to 'Abilities - Normal' list, or add them via trigger, will have level greater than 0.