I've heard lots of complains that skill cards are now useless because all they do is lower the mana cost by 2 or so. I have to admit that I was pretty annoyed too at first, because I had bought Nightmare Card +1 for 200k in e3 and now it says that only effect is -2MP. To find out is there really hidden effects, I did some testing with my lvl 37 Jlv 30 DM using Life Leech Card +1 and Nightmare Card +1, which both say that the only effect is -2MP. In tests I didn't use any buffs or chips, just my equipment and skills.

In Life Leech test I casted Life Leech lvl3 ten times to Blue Chest Aves Juniors north of Horizen with and without card. Without card I did 550-596 damage and recovered 181-196 HP. Average damage was 575,4 and average recovery 189,5. Then I equipped card and killed ten more Aves Juniors. The damage now was 581-638 and HP recovery 197-216. Average damage was 612,1 and recovery 207,5. As you can see, there definetly is a hidden effect that is not mentioned in this card. On average the spell did 36,7 more damage and I recovered 18 more HP when using card. Or it did 6,4% more damage and 9,5% more recovery. This test can't tell if the hidden effect is based on percentage or if it is fixed amount of damage/recovery it improves. To know that I should do some more testing.

In Nightmare test I casted Nightmare lvl4 to 10 Skell Warriors of Pain (lvl 35 mob)in Dead Spirits Altar with and without the card. Here the result was that in both cases it succeeded in 7 cases. This test cannot be relied on because of limited amount of casts. To get a reliable result the spell would have to be cast atleast on 100 mobs and then equip the card and repeat the same thing. That is why I believe that there is hidden effect in this card too, altough the test can't prove it. Small succeeding improvements can't be found in this small test.

Conclusion: Cards have hidden effects. Don't trust what is written on the card. They may do much more good.