Alan Williams hypothesized that the ULFBERH+T swords may have been made from Asian crucible steel on the basis of high carbon content and microstructure. The popular press then ran with a corruption of that theory - that ULFBERH+T swords were the best - luxury goods of their time - and the other variations were knockoffs.
However, far from being the Viking Super Sword, the +ULFBERH+T inlaid swords were likely from a particular workshop that used higher carbon steel and corrupted the earlier existing +ULFBERHT+ inscription and there may be a metallurgical reason so many are found broken.
For me, Ingo Petri pretty much shot down the Viking super sword concept in 2015 at the Solingen
Sword: Form and Thought conference. Here is a more recent paper by him explaining his reasoning:
https://www.academia.edu/68589269/VL...nd_manufacture. (The formatting is better if you download the pdf from there.)