I did find a site for this a while back that quoted a Roman source. I can't remember if it is Caesar or tacitus, but the Southern tribes of Britons were described as being more "like the Gauls" than their northern counterparts, who were always in a constant state of war with each other.‘the fighting between the northern tribes was considered to be a lot fiercer, as they had fewer resources and raided each other a lot more’ interested to know what your evidence is?
In addition Scotland is known for its unique Iron Age defences... Such as Brochs and Crannogs. (a Crannog being a settlement that was literally built in the middle of a Loch, a broch is a round almost tower-like structure made of stone suspected to be used as a store and defensive structure to prevent raids) Also some areas of Scotland had far less wood and Celtic hillforts were also made of Stone when that resource was nearby... But we do not have much information about the Northern Celtic tribes other than what Tacitus tells us about the Battle of Mons Graupius, and given he was related to Agricola the General who commanded the battle his account is bound to be exaggerated and flawed.
At any rate the Romans declared that all of Britannia was subdued after victory at Mons Graupius, in reality the surviving Caledonni and the Picts melted away into the countryside after their defeat while the Romans chose to pull back south rather than hold on to the North... Presumably there was no resource of value to keep them interested in maintaing a garisson that far North.
When Hadrian's wall was built later, Pictish raids on the wall were commonplace, and the Romans often engaged in strikes north of the wall and has some success in limiting the raids. But the Pictish problem remained a constant threat until the year the Romans left.
Bookmarks