Hi! Good question, if you have a 40% buff for Ninjutsu on a ninja (for example), it will affect the Ninjutsu stat corresponding to the ninja. So for a ninja who's Mystery is affected by both Taijutsu and Ninjutsu, the Ninjutsu buff would only affect the Ninjutsu portion of the Mystery, corresponding to the Ninjutsu stat of the ninja.

This means that a 40% Ninjutsu buff on a Mystery that is both Taijutsu and Ninjutsu would in most cases be less effective than the same Ninjutsu buff on a Mystery that is purely Ninjutsu because the buff only affects theoretically half the stats (on a Taijutsu and Ninjutsu Mystery).

Hi! Your statement about ninjas having different base stats for Ninjutsu and Taijutsu is correct (Example, some have a higher Ninjutsu stat, while others have a higher Taijutsu stat). This is why the explanation I gave above outlines that the 40% buff for Ninjutsu on a ninja is dependent on the "Ninjutsu stat" corresponding to the ninja. It is a buff that affects the Ninjutsu portion of the mystery based on the corresponding "Ninjutsu Stat" on the ninja.

Which is why, the answer to the question is that a "40% Ninjutsu buff on a Mystery that is both Taijutsu and Ninjutsu would in most cases be less effective than the same Ninjutsu buff on a Mystery that is purely Ninjutsu because the buff only affects theoretically half the stats (on a Taijutsu and Ninjutsu Mystery)". These stats are dependent on the corresponding value for the Ninjutsu and Taijutsu of a ninja.

The reason for my answer is because for a mystery that is both Ninjutsu and Taijutsu, there will always be a percentage of the mystery that is part Ninjutsu and part Taijutsu. In this example, a Ninjutsu and Taijutsu mystery will always have a value that is more than 0% for both Ninjutsu and Taijutsu. This means that a 40% Ninjutsu buff on a Pure Ninjutsu Mystery which is 100% Ninjutsu, will in most cases benefit more from the buff than a mystery that is both Ninjutsu and Taijutsu.

Example Calculation: X%[Ninjutsu]+X%[Taijutsu] = 100%

Let's say theoretically, Ninjutsu and Taijutsu is divided by an equal amount of 50% each for a Ninjutsu and Taijutsu mystery. For example, if the Ninjutsu stat for a ninja is 1000 and Taijutsu stat for a ninja is 500, the calculation would be (50%x1000)+(50%x500) = (0.5x1000)+(0.5x500) = 500+250 = 750 (500 portion of the 750, is the theoretical value of Ninjutsu)

On the other hand, if the mystery is pure Ninjutsu. For example, the Ninjutsu stat for a ninja is also the same 1000 and the Taijutsu stat for a ninja is also the same 500, the calculation would be (100%x1000)+(0%x500) = (1x1000)+(0x500) = 1000+0 = 1000

Since the first theoretical calculation accounts for the fact that Ninjutsu stat affects a mystery partially, while the second theoretical equation accounts for Ninjutsu stat for 100%, one can see that a 40% Ninjutsu Buff would equate to a higher value on Mystery that is pure Ninjutsu compared to the Mystery that is both Ninjutsu and Taijutsu.

Final Example Calculation (based on Example above):

Theoretical affect of Ninjutsu Buff on Ninjutsu and Taijutsu Mystery: 40% of 500 = 200 (to be added onto base value)

Theoretical affect of Ninjutsu Buff on Pure Ninjutsu Mystery: 40% of 1000 = 400 (to be added onto base value)

Hopefully that clears up some of the misunderstanding =p

We are driving home a very similar point but just in different words =p