It's no where near the best game out there, but it isn't as bad as it might seem initially. Hero chances are low, but they DO drop from all 3 books, bosses have a chance to drop in its entirety when clearing a stage, and finally, if you do spend money on this, the VIP is best, use the daily gems for insta clearing/more meat. (Never did spend a dime, still completed the main campaign and got a by 4* hero collecting souls)
Gold and Gems do not yield Heroes Spending either forms of currency in game is meaningless. You cannot obtain heroes doing so. You also cannot find heroes through grinding. The success rates are very low. Don't waste your time.