Tribal models of warfare are structured differently and rely on much more lightweight and distributed coordination models that require very little non-local communication. They also tend to be relatively homogeneous (all infantry or all light cavalry for instance) and do not typically practice complicated large area/time scale maneuver models (i.e, requiring coordination across more than 100 miles and a few days for instance).

Their code of honor is also more about showing individual courage than winning.

All this can be achieved by loose coordination among sub-Dunbar units. A council of tribal chiefs the night before the battle can be enough ("your village attack from the hills at dawn, we'll attack from the river").

All these are disadvantages against "civilized" armies under normal conditions. But when the civilized army has morale/discipline problems and/or does not know the terrain well and/or lacks tactical agility (i.e. a little confusion rips their complicated coordination to shreds), the tribal model has very significant advantages. Especially if the tribal side is on the defensive and only needs the other side to fail to win (i.e., has no goals itself for occupation/subjugation, just some loot).

The comparison is almost exactly analogous to waterfall vs. agile models of s/w development. Agile models don't scale well beyond a couple of dozen, but in chaotic conditions, they still beat waterfall.

There's a reason major civilized armies were routinely beaten by tribal ones in history, a phenomenon that continues to this day.