I was a backer for the 5e Thule setting book on Kickstarter, so I've had the PDF for a long time and the hardcover for a few weeks now.
The terminology/5e translation is pretty solid, though there are a few minor annoyances ("tactical advantage" used in place of "advantage," stuff like that) and a couple of minor editing oversights (we are told that Thule has no kobolds in one section, but kobolds are listed as speakers of Draconic in another). Overall, though, I'd say the book is incredible.
It's true that while the authors make noise about the rarity and danger of magic, this idea is not mechanically supported within the rules presented in the book. This doesn't bother me personally; I use many ideas from the 3rd party Pathfinder supplement "The Spider God's Bride," and everything works out in my campaign. But be forwarned that while the Thule book itself is designed so it can support traditional gaming styles, and some adjustment will be needed to run it as a low-magic and/or gritty setting mechanically.
I love the narratives. They're really more 2e kits than anything else in my opinion, and carry with them all the potential complications that implies. Many of them give characters followers at 10th level, and the book has a little bit of advice on how to make that work.
There have been a few threads on these boards discussing the setting and adapting it to a low-magic paradigm. There is also a fledgeling Thule forum Morris linked to a while back, which has some more discussion on houserules, grittiness, and other resources.
Honestly, the book is worth it for the fluff alone, in my opinion.