Bersekers obviously were something in Norse culture but what? Were they really completely naked save for an animal skin or is that just pop culture? And did they really work themselves into a frenzy were they would attack friend or foe? Seems like a liability rather than an asset.
Yes, there was probably warriors called berserkir (in Old Norse) in Viking Age Scandinavia, but it does not necessarily means that the popular images of the "berserker" in modern fictions like the recent film "Northman" reflect the social reality.
While more can always be said, I summarized the recent critique, focusing on the possible difference between actual primary texts (especially pre-Christian poems) and these popular images, recently in the following threads:
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As for the popular representation of the Viking/ berserker without shirt, I'd also recommend these answers by /u/sagathain and /u/Steelcan909:
In addition to the excellent answers already given, there's also the Christian chronicler perspective on Berserkers, which muddles things even further.
The Christne Saga (Icelandic saga likely written in the 12th century) narrates a few encounters between berserkr and the Christian missionaries sent to convert Iceland in the late 900s. Here's a few (translations mine so there's likely a better one out there somewhere!):
"Berserkrenn mælte; 'Eige muntú þora at berjaz við mik, ef þú sér íþrótter mínar. Ek ganga berom fótom um eld brennanda, ok ek læt fallaz berr á sax- odd minn; ok sakar mik hvárke' Þangbrandr svarar; 'Goð mun því ráða' Þangbrandr vigðe eldenn, en gerðe cross-mark yfer saxeno. Berserkrenn brann á fótom er hann óð eldenn. En er hann fell á saxet, stóð í gegnom hann; ok feck hann af því bana."
[The berserk said; 'You will not dare to fight with me, if you see my feats. I go barefoot over burning fire; and I allow myself to fall on a bare knife-point; and neither does any harm to me.' Thangbrand replied; 'God shall decide this'. Thangbrand consecrated the fire, and made the sign of the cross over the knife. The berserk's feet were burned when he waded through the fire, and when he fell upon the knife it stabbed through him; and he received his death this way.]
Another missionary named Frederick had an earlier encounter:
"Þar kómo berserker tveir, er Haukr hét hvárr-tvegge; þeir buðo mannom kúgan, ok gengo grenjande ok óðo elda."
[there came two berserks, who were both called Hauk; they challenged many men, and went around screaming, and waded through fire]
In that particular encounter the berserks were defeated via Frederick sanctifying the fire, causing it to burn them, and the watching Icelanders immediately took advantage of this and killed the two berserks.
The attributes of the berserkers above are likely exaggerated for the impact of the story, especially since this was written by a post-conversion Icelandic author. Since Iceland converted via legal methods, berserkers as lawless crazy people fit nicely into two narratives: scary pagans and scary law-breakers.