If you look at a relatively simple text, say the Ynglinga Saga, you'll see that historians disagree about how to treat it.
Some historians think that it preserves an authentic tradition (which is very different from saying it is true). Some think that's it's a pile of rubbish, cobbled together to make a continuous narrative. Probably most focus on the narrative break at Ingjald, which suggests that Ingjald's story might have been grafted onto an older tradition and that the later Norwegian kings wanted to get the prestige of being connected to the Ynglings.
When I said that modern historians regard the truth about genealogical relationships to be "unrecoverable", I mean that literally. In the Ynglinga Saga historians don't spend time worrying about whether the genealogy is correct. The important question is whether it is an authentic tradition.
Many historians have grave doubts about the main premise of genealogy -- the idea that anyone can string together a bunch of different documents over hundreds of years and come up with a "family tree". Yes, each generation seems to be connected, but the overall effort is open to doubt. If you're 70% or 80% sure of each generation, by the time you get back a dozen generations the overall certainty level is very low.
So from a historians point of view, anyone who gets back to a figure mentioned in the sagas has already indulged in too much wishful thinking. They would say, forget the sagas and concentrate on the real weaknesses in that chain. As an example, I had one professor who, having seen the evidences, nevertheless refused to accept that Queen Elizabeth II could prove descent from William the Conqueror with any reasonable certainty ;)