There were still battles between the Byzantines and celtic/germanic tribe descendants. They even wrote a whole chapter in their Strategikon (a great book if you haven't yet read it, written in the 6th century by the Byzantines themselves, which digs deep into their formations, tactics, logistics and the characteristics of various people they had to fight against, from Persians to Scythians to Slavs to Germanic peoples).
Chapter 3: Dealing with the Light-Haired Peoples, Such As the Franks, Lombards, and Others Like Them
“The light-haired races place great value on freedom. They are bold and undaunted in battle. Daring and imperious as they are, they consider anuy timidity and even a short retreat as a disgrace. They calmly despise death as they fight voliently in hand-to-hand combat either on horseback or on foot… Whether on foot or on horseback, they draw up for battle, not in any fixed measure and formation, or in regiments or divisions, but according to tribes, their kinship with one another, and common interest… Either on horseback or on foot they are impetuous and undisciplined in charging, as if they were the only people in the world who are not cowards. They are disobedient to their leaders. They are not interested in anything that is at all complicated and pay little attention to external security and their own advantage. They despise good order, especially on horseback. They are easily corrupted by money, greedy as they are. “
“They are hurt by suffering and fatigue. Although they possess bold and daring spirits, their bodes are pampered and soft, and they are not able to bear pain calmly. In addition they are hurt by heat, cold, rain, lack of provisions, especially wine, and postponement of battle. They are easily ambushed along the flanks and from the rear of their battle line, for they do not concern themselves at all with scouts and the other security measures. Their ranks are easily broken by a simulated flight and a sudden turning back against them. Attacks at night by archers often inflict damage, since they are very disorganized in setting up camp…”
“The light-haired races place great value on freedom. They are bold and undaunted in battle. Daring and imperious as they are, they consider anuy timidity and even a short retreat as a disgrace. They calmly despise death as they fight voliently in hand-to-hand combat either on horseback or on foot… Whether on foot or on horseback, they draw up for battle, not in any fixed measure and formation, or in regiments or divisions, but according to tribes, their kinship with one another, and common interest… Either on horseback or on foot they are impetuous and undisciplined in charging, as if they were the only people in the world who are not cowards. They are disobedient to their leaders. They are not interested in anything that is at all complicated and pay little attention to external security and their own advantage. They despise good order, especially on horseback. They are easily corrupted by money, greedy as they are. “
“They are hurt by suffering and fatigue. Although they possess bold and daring spirits, their bodes are pampered and soft, and they are not able to bear pain calmly. In addition they are hurt by heat, cold, rain, lack of provisions, especially wine, and postponement of battle. They are easily ambushed along the flanks and from the rear of their battle line, for they do not concern themselves at all with scouts and the other security measures. Their ranks are easily broken by a simulated flight and a sudden turning back against them. Attacks at night by archers often inflict damage, since they are very disorganized in setting up camp…”