Norse: Strong Archers, Okay spearmen
North Celts (Irish/Picts): A lot of skirmishers, probably the best but skirmishers aren't generally powerful units. Seemingly weaker melee soldiers (horrible shields)
Angles/Saxons: Closely resemble Norse
Frisians: No archers. Alright Cav, "meh" tier melee
South Celts (Britons/Welsh): Also have skirmshers, though I'm not sure on their melee units
It's important to note that top tier units are usually armored in chainmail and perform pretty closely in melee fights.
Generally mercs are also going to outperform their faction equivalents
Note: There might be some errors here as this is all done from in game memory