The incompetence of the Yunkai lords and the ineffective nature of their slave soldiers is on full display to readers and the in world characters alike. Its just so over the top though that I think GRRM is trying to get his readers to underestimate the slaver coalition. Tyrion takes stock of their forces near the end of ADWD (the slingers and New Ghis armies) and notes that they aren’t up to par with their enemies soldiers, but I think there is a good implication that there is iron there as well