I'd been hoping for a train wreck, but I was disappointed. I'd heard that Thor fans hated it and that Ennis was pissing on superheroes (again), but it seemed fine to me. The plot's extremely straightforward, but I don't object to that. I enjoyed it. I'm speculating now, but I'd guess that Marvel fanboys dislike this series because:
1. Ennis writes Dr Strange as John Constantine and ignores how magic works in Marvel comics. He also doesn't have Wong (with no explanation) or his usual magical arsenal (e.g. the Cloak of Levitation). He sorts out the situation (albeit by getting others to do the fighting for him), but he's a dick and has Constantine-a-like dialogue like "shaft the poor buggers royally". This is distracting, but I managed to find it funny.
2. Some continuity errors, apparently. I didn't care about those.
3. At least at their lower end, superhero comics are comfort food. Superheroes win fights. That's the job description. That's what the audience expects to see. Here, though, Thor gets smashed into paste in issue #2 and it's even more ugly and one-sided than you're imagining, after which an Avengers team gets trashed offscreen. Captain America, Iron Man, Scarlet Witch and the Vision become mooks.
4. It's probably not canon. Ennis says it is, but its events aren't mentioned in any other Marvel comic, even though Viking zombies sack Manhattan, flatten all superheroes they meet and kill so many people that they're effectively an undead nuke. (The Viking zombie leader got namechecked in the main Thor series in 2018, but that's all.)
5. The Vikings are ultimately stopped by... yes, by Thor, but also three ordinary warriors from history. Dr Strange chose them for magical reasons and gave them strength boosts. There's a woman who'd wanted to be a Viking (and presumably hadn't heard about all the rape), a deranged bastard who burns heretics at the stake and a German fighter pilot from WW2.
On the upside, though, Ennis takes Thor himself seriously. He does all that Shakespearean dialogue and makes him completely noble and heroic. Norse mythology is presented literally (e.g. Valhalla), which is quite interesting. Thor gets pulverised and spends most of the story listening to Dr Strange, yes, but he comes back and has a cool victory at the end. He even made me laugh once, with "Thor has his doubts about this one." (There's also a panel where he says, "Art thou another that would mock the Odinson's manner of speech?")
Also, the Vikings are unspeakable. Imagine everything the Vikings really did do. Imagine Ennis writing that, with no holds barred, right from page one. They're beyond disgusting even before they visit New York and go beyond even what I'd been expecting.
There's even a comedy cutaway to the idiot in the White House. Given the publication date, this will be George W. Bush.
It's Ennis using Marvel MAX to beat up superheroes again. It's in bad taste and has a tidal wave of gore. It's pretty much what you'd expect from that. The nearest I have to a criticism, personally, is that I'd have liked the series to last for six issues, not five, for a better showcase of the three historical fighters. (They're quite amusing, especially how morally compromised they are, and I'd have liked the chance to spend more time with them.)
Personally, I'd like to see Ennis write Thor again. I'd be interested in seeing how his anti-religious instincts interact with a heroic god and the very odd specifics of Norse mythology.