It's physically a beautiful book and that's a definite point in its favour.
As others have said, the book seems to use somewhat inaccurate information at times, or to cite "some scholars" or "some sources" with no specificity. The critiques others have stated, to my mind, are all reasonable. I caught several weird spellings, and I'm not even Scandinavian.
Occasionally throughout the book, Guerber will make some bizarre explanation for a myth - she's particularly fond of claiming that swords represent the sun - and treat it as if it's pure common sense. Because of this, I never took her analyses at all seriously. There are also multiple points where she'll say things like "days obviously represent months" and ended up coming up with about 6 different explanations for winter which all seem unreasonable to assume to be accurate when her comparisons are so frail.
I think it really says a lot if it takes me almost two months to read a book. I'm not exactly sure of when I started reading it, but it was definitely mid January. Typically, even taking more than a week is reserved for absolute bricks of books (like Lord of the Rings), and something that's only a couple hundred pages taking two months is not a good sign.
That said, the chapter that really irked me was the last one, where she compares Greek and Norse mythology (or claims to!) to evidence a common origin. I have a lot to say. tldr; it makes the whole of the rest of the book worse by being so incredibly incompetent.
I'm not being at all conclusive here, but there's just such a long list of ridiculous flaws in her comparison that even a cursory summary of them looks enormous. It literally reads like one of Ben Shapiro's Gish Gallops, where the aim is to say so many things that are inaccurate or ridiculous that any academic critic just gives up.
Firstly, she says she's using Greek mythology but primarily uses Roman terms, which seems minor, until we factor in that the Romans had major splits from the Greek pantheon, and that it does change some of the parallels. For instance, the Greeks had a total of 13 primary gods in their pantheon, and for a parallel to really make sense, one of the Norse gods would need to be ejected, parallel to Hestia, and another ascend, as parallel to Dionysus. Additionally, Athene is a badass warrior bitch, like Brunhild, but she has two Roman counterparts; Bellona is also a badass warrior bitch, but Minerva is a shrivelled old maid. Comparing Brunhild, a badass warrior bitch, to Minerva, when there are two immediate and far better comparisons (one of which belongs to the myths she claims to be analysing!) is absolutely absurd. The only reason I can think of for the conflation is that Pluto specifically has a wealth domain that Hades does not, and her claims about dead people being dwarves can at least obfuscate minimal credibility for a god with links to wealth; did she really sacrifice her entire argument for one tiny comparison?
Many of the comparisons come down to "two obviously not at all parallel Gods both owned x"; Poseidon and Loki are transparently NOT parallels, sheep are not horses, pigs are not chariots. Almost all of her comparisons involve comparing tiny aspects of one god to tiny aspects of several others with no link between these comparisons. Many of the comparisons are simple coincidence; many mythologies feature inter-related Gods, many mythologies have lighter hair representing sun or light gods, etc. Several of her points could be entirely shown as irrelevant by adding that the Christian God also fits the parallel she's making; she's having to derive such broad comparisons that the comparison doesn't even make sense any more. At one point she claims Odin, Var, and Veli are parallel to Zeus, Hades, and Poseidon, when surely Odin, Hel, and Ram are the parallel gods and are very obviously not a trio. Earlier Guerber went out of her way to qualify how large a ship was, but then in her intermythological comparison qualified its size by saying it could carry all of 12 people! Cyclopes are distinctly linked to Poseidon, not Zeus, and have absolutely nothing to do with sacrificing one of Odin's eyes, and a weaponry link is feeble. Ragnarok is in the future while Troy supposedly happened long in the past from the Greek civilisation. These comparisons regularly bring in completely irrelevant points which don't have a parallel, or will even describe how the stories aren't at all related as an explanation for how they're related. There are some genuinely interesting parallels present, but it's a small minority of the supposed parallels that Guerber makes, and they're absolutely swamped in nonsense.
Geurber makes easily demonstrably wrong claims about Greek mythos too; Athene's eyes are described as "χλορος / chloros" in the Iliad which is an ugly yellowish-grey-green colour (and source of the word "chlorine", which is not a compliment), not a "beautiful blue". She randomly sprinkles in just a couple of Greek names, just enough to be frustrating when she's using Roman names for most of the gods (despite claiming she's examining Greek myths...); the worst example is that she specifically switches to using Roman "Proserpine" when she switches away from Roman "Pluto" (which she'd been primarily using) to Greek "Hades" in the same sentence! The Greeks had 5 wind gods which is obviously entirely divergent from a singular bird causing wind. If she's going to parallel tears creating gold to tears creating gold why does she randomly weaken her argument by putting tears creating white or purple flowers in the middle as part of the comparison too?
I feel like it's very telling about the quality of her parallels that Artemis and Ares are barely mentioned, Hestia is not mentioned, Aphrodite and Demeter are frequently conflated, but Hermes is a primary focus. Why on earth would Dionysus be included? He was transparently, even in the myths, a later addition to the pantheon that couldn't have been a common derivative. Heracles was a minor god, why is he getting more focus as Guerber's "proof" than all the goddesses combined? I can see why she's using the Roman names - the Greeks are far too socially progressive for her, judging by who she's ignoring! She talks about Heracles "becoming" a woman as a parallel for Thor dressing as a woman once, which feels like comically absurd phrasing when she's already including Dionysus who was transformed into a little girl as a baby and became a man as he grew and was therefore a god of being transgender (and of gender nonconformity generally)?
The conclusion doesn't even necessarily make sense as a sole possibility if there *were* parallels, because the ancient Norse and the ancient Greeks both honoured travel as cultures, and as sea-faring peoples, it's not at all unreasonable to think that either group could have found influence from the other after forming their own myths. You only need one group to have previously been Greek influenced and then be conquered by Vikings for a lot of Greek influence to spread. I'm saying this while thinking her conclusion that European mythology has a shared origin is correct - she's just made her argument so poorly that I *want* to play devil's advocate.
That's before baring in mind that comparisons like this often exclude enormous amounts of the religious system, ignoring anything that's not convenient wholesale. Guerber gives the convenient excuse that she's only mentioning her "main points" - to which I say why did she include so many absolutely rubbish comparisons then? Surely a better (and more academically plausible) excuse is to say that anything unique must have been derived post-divergence?
The biggest part of all this that frustrates me, however, is Guerber comparing Freyja (spelled Freya for some reason) getting an involuntary haircut from Loki to Zeus' horrific sexual violence. As a sex violence survivor, this is DEEPLY offensive. I cannot take her seriously if she genuinely believes that the singular short-term violation of an involuntary haircut (and it is a violation, I'm not dismissing that) is in any way comparable to multiple women being forced to carry their aggressor's children and being deemed Zeus' "wives" by many accounts. It left me less willing to believe the rest of her argument and made her entire book less credible if she's so out of touch with the comparisons she's willing to make. There are better comparisons already present!
She frames this chapter like it's been her thesis throughout the entire course of the book, despite absolutely no foreshadowing. This chapter being included makes everything else significantly worse. I had naturally been making parallels previously, as well - so the chapter wasn't even needed! If she's this completely incompetent about her analyses and this incapable of basic understanding of what things actually mean and this incredulous about her audience's reading comprehension, I can't believe her for any of her previous discussions - and I was already deeply sceptical! Who on earth decided that the best way to publish this book was to keep this chapter in the manuscript without criticising it?