yes, some r cards suck real bad and literally boost or drop cards stats by a flat 300 even.
mix faction will be very common later, as more cards release, all types stats up, all types stats down, all types damage reduction, etc, in the future.
the 10% atk up or down versus a faction is determined from leader vs leader. if your leader is taimanin and you face a demon leader, you gain 10% stats and they lose 10% stats, to the whole team iirc. regardless of what faction the rest of the team consist of.
as for factions, USF is really bad at launch. they don't have any HR or lower 25% boosters. the best booster 25% the gunslinger eye patch girl who boost by 22% lol. they also have the worst of the SR's. at least their UR and LR are good, but no one is likely to have more then 1 lol.
edit: way to go dude! if you have her in your deck. you get 15% more points earned in the tower event.