No guys if you have 90 lvl hero with no equip and 7 stars and noabilitties, the system calculates your overall rating and puts stronger enemy in cof. So you should wait as you can to instant fully promote your hero when he is ready to fight. I have 3 unactive cards because i dont have resources to train them fast. + you need tonns of gold for abilities .
If i open my cards i will have abuze with no actual strenght i hope you catched my point of view))))