Overall, I don't have a problem with the card mechanics, but I do think there could be a buff to most, if not all of them. Despite the OP singing praises about the Arrow Card...I've hardly noticed any effect when I use it on myself. Same could probably be said with most of the cards....
It's all relative. In general, the cards are sort of weak. If you're in an 8 man party and you give 1 of the 4 DPS a 15% arrow card (Royal Road boosted), overall, the party's DPS isn't going to change much.

However, compared to the other cards, this is still good, sadly.