I'd hazard a guess that people didn't hate the 4e FR books because the books themselves were necessarily bad. It's not like they were poorly edited or badly written or of less quality than other books. WotC is pretty darn good at producing high quality books.
The reaction to 4e FR was more due to the massive changes they tried to ram onto the setting that invalidated so many people's campaigns. It wasn't how the book was written that was the problem, it was what was written that was the problem.
That doesn't seem, as much, to be the issue here. No one seems to be complaining about the canon that is presented, more about how the canon presented only covers a part of the setting. But, my point is, it says that right on the cover of the book. I wouldn't expect a Sword Coast Adventurers Guide to talk about any other area of the setting other than the Sword Coast. Considering that we have three complete campaigns now, and likely a few more coming in the near future, it shouldn't be a surprise to anyone that most of the information is covering this area.
I also wonder if some or a lot of the material in this book comes from the Elemental Companion guide that was cancelled (er whatever) last year. IIRC, the class elements from the Elemental Companion were about on par with this book. I'm going from memory here, but, wasn't there like 20 or 30 pages of class material in the pdf? I'm wondering if the plan wasn't to have SCAG originally be the Elemental Companion, and then this book would expand on that book.
Thing is, we're a year (ish) after the Elemental Companion and people, after all this time, really were expecting a lot more from this book. Had the Elemental Companion book come out on time with Princes of Elemental Evil, complete with the setting material from SCAG and the the SCAG comes out now with expanded material, I wonder if people would be a lot more happy about the two books.
And, I wonder, perhaps, if we'll see more of these Companion Guides released with each (or maybe every other) AP in the future. Of course, this is pure speculation on my part and I know WotC doesn't dare say anything because people will take anything they say as a contractual guarantee. :/