Card skills play a huge role in determining victories or defeats. Generally match outcomes aren't determined by card stats as much as they are by card skills. In Marvel: War of Heroes the skills are broken into attack and defense orientated skills that target allies or enemies.
The power of the skill is determined by two things, first is the descriptor of the ability. For example, an ability offering a Notably buff will be weaker than a skill offering a Extremely. The second thing that determines a skills power is the level of the skill, the higher the level the stronger the effect of the ability.
I have developed a theory on buffs based on my testing as well as applying theory from Rage of Bahamut skills.
There are five base power levels: Partially, Notably, Remarkably, Significantly, and Extremely
The effects are: Intensify or Degrade. Intensify being boost, degrade being debuff/hit. I will note whether its intensify or degrade it has no bearing on the percent of boost.
The effected are: Specific, All
Stats effected are: Atk or Defense (AorD), Atk AND Def (AD), or Self (S)
Feel free to let me know if you've experienced different results:
Partially Notably Remarkably Significantly Extremely
Specific (AorD): 5% 8% 12% 15% 20%
All (AorD): 3% 6% 9% 12% 15%
Specific (AD): 3% 6% 9% 12% 15%
All (AD): 2% 4% 7% 10% 12%
Currently all self buff stats are unknown, but if I were to guess I would say that they cap at around 50% at the highest specific self buff, then drops to 40, 30, 25, 20.
Self Attack AND Defense buffs I would guess would be 5% less then their specific counterparts.
Feel free to let me know if any of my info is inaccurate.