What missing wrecks leave uncertain about major Viking longships

How much does missing evidence limit knowledge of major Viking longships such as the Long Serpent?

Narrative history survives, but missing wrecks and architectural evidence leave construction and alteration details uncertain.

Considerable narrative history survives about the Long Serpent, while important construction details remain uncertain. The largest ships might have retained familiar clinker-built, low-freeboard longship forms or acquired alterations that brought them closer to cogs. Resolving that uncertainty would require a surviving wreck or substantial architectural evidence. Wooden construction and unsuitable wreck environments make such evidence difficult to recover. The associated king's fate also remains uncertain: survival amid the confusion of battle and a legendary leap overboard are both considered.