The problem was 'pain'. This was a serious issue because they all hated pain, and it was certain that pain would occur. This was one of the reasons why they decided to use races that caused them the most pain, although it was shameful to them. They accepted it as a consequence to gain more information. They had no choice but to adapt.
However, what seemed like a simple thing to HUMAS turned into hell for all the other races that had harassed the HUMAS. As fate would have it, they were perfect for the experiment that the HUMAS were conducting. Since the HUMAS themselves were weak, they could only use races that were equal to or weaker than them. Even so, these races were still better experimental subjects than the HUMAS themselves. They divided all the races they knew and realized that the truly strong races never harassed or killed them unless they were truly annoyed. This often happened since the strong races were the ones with more information.
It seems that the stronger races saw it as beneath them to intentionally mess with the weaker races, but this didn't matter to the HUMAS. The only reason they didn't experiment on the stronger races was simply because they were too strong for their current capabilities. Through their experiments, the HUMAS discovered that certain races were better subjects than others, leading to the specialization of subjects.