A power is unbalance by its very nature. That is why I have made the player choose between a limited frequency or have to water down their power's usefulness. This is to assure that the power doesn't dominate the game. This is the "It doesn't work that way" that we often see in the movies.
Vulnerability is an entirely different animal. This is a roadblock from using your ability. Most books and movies wind up having a number of these, but don't really have a catch-phrase to quantify it. The most predominant restrictions are things like magical items or item focuses that can be taken away, kryptonite, concentration, a ritual, emotional mojo that gets out of whack with psychological wallops, or other things like that.
That is why the default is "Mystical and unpredictable" basically it is "I don't know why it doesn't always work."
Let's see, here are some ideas I can see that could fit.
-Physical- Cause (or cannot have) pain exhaustion or injury, requires sleep, being wet, etc...
-Psychological- Emotional state dependent, can only retrieve a memorable event, cannot worry or focus on the future, etc...
-Situational- Ritual, concentration, a special item to help initiate the power, moon must be aligned, etc...
-Material- Doesn't work in the presence of ash, silver, rare or unique items, diamond dust, etc...
-Retrieving end- can only return once, cannot see own reflection, etc...
Don't forget that the easier to disable, the easier the recovery. However, the harder to disable, the harder the recovery.
However, this doesn't mean that you have to choose Minor, you could choose one of the other two, and basically it would mean "it doesn't matter how many times I look, the information just isn't there" and thus it becomes irrelevant to have it be continuous.