For mercenaries, making money is the top priority. As an employer, you just need to tell them what the target is, pay them if the mission is completed, and forget it if it's not.
But for someone like Satan who's just temporarily hiring some mercenaries and then going to war with them, things are a lot more complicated.
The mercenary ranks are a mixed bag of backgrounds, each with different training and combat experiences; it's not easy to forge such a group into a cohesive unit.
However, the most troublesome aspect isn't the problem of capability and coordination, but that there are no good men among mercenaries; basically, every mercenary is a troublemaker.
Inherently a trouble-making bunch, Gao Yang had recruited people who had made some name for themselves in the mercenary circles. To put it bluntly, these guys were all elites, and to get such a group of proud and haughty people to obey orders, just paying them was obviously not enough, far from enough.