How are pastoralists more "flexible" than agriculturalists?
They can move out of their homeland or organise military campaigns more easily than agriculturalists, which being bound to their soil and harvests. In Mesopotamia, we have the description of such a situation, which the people of Tripolye-Cucuteni experienced in Eastern Europe as well, in all likelihood: At first the pastoralist tribals came to burn the fields down and take the crops and animals. Whenever the Sumerians came with their army, they came too late. The damage was done, and the raiders were gone. The borderzone clans did so for many years, until the defense and supply of the inhabitants was completely ruined. In the end, they just needed to take the what remained, physically and mentally they were broken already.
If you think about the huge settlements of Tripolye-Cucuteni, they were very hard to storm by any sort of raiding party, even larger alliances of steppe warriors. But they always had allies on the steppe, which kind of protected their front yard. When the pressure from the East pushed their allies into their territory or away, they had to defend on the steppe, and that was hard to do. Because the enemy was more mobile and quicker. The pastoralists can hurt the farmers all day, but the armies of the farmers can't do the same as easily.
And even in a worst case scenario, the pastoralists can flee with their flocks, but the farmers can't.
Even worse than regular farming is civilisation, because the more complex a structure, the harder it deals with damage and downgrading. Like what do people of an urban centre do, if the farmers from around their city being captured and killed, or fled, or turned against them? The tribal, pastoralist warrior can just move on, doing what he did before, but the urban people mostly died.
So the disadvantage the farmer already got, being multiplied by "civilisation" and urban settlement. It's only an advantage if it can scale up or create advantages which the more simple, small scale and clan based groups can't copy. Like top-level, next-level military organisation and equipment. But on a smaller scale, once the organised, large scale resistance of a state being broken, the local inhabitants of a civilisation being usually just easy prey for tribal people.
This is also why in the Balkans Vlachs and Albanians did fairly well after the collapse of the Roman Empire, but other Romanised people did not.
The pattern in Mesopotamia and Tripolye-Cucuteni being pretty similar. They tribal groups could have many lost battles and setbacks, but the more settled down and civilised groups just one. One bigger lost battle or setback and they were done. Because there is no way to come back once hte complex structures, the fields and harvest being lost. The fortified centres on their own are not defensible without supplies.