To be honest, while some mementos seem to be a no-brainer (like attrtibute points), I still don't get the idea behind some of the other mementos.
For example, Voltaire's Edits provides +50 culture when you spend a Cultural Attribute Point and is unlocked on Level 5 of Friedrich, Baroque. I always assumed that the Level 5 mementos are quite powerful, but I don't see much value in this. Is this supposed to be used in combination with the memento granting an attribute point for an initial boost to culture? This may seem useful at the start of antiquity but seems to be rather weak later on.
The same applies to all the other +yields whend spending an attribute point. They seem to provide a very low yield in view of the number of attribute points you are expected to get in any age, yet they all require level 5 of a specific leader.
Or consider Bifocals (Franklin, Level 2, +50 influence after researching a civic or tech mastery) in view of Globus Cruciger (Charlemagne, Level 5, +50 influence when spending a dipomativ attribute point). Here the Level 2 Bifocals seem to be more powerful than the Level 5 Globus Cruciger, since you can get a lot more masteries than attribute points. So I don't get why those momentos are level 5, or do I miss something here?
I don't mind that some mementos are more useful than others, but shouldn't the higher level ones be stronger in general?