Shadowrun 6th is not mediocre, it is polarizing.
I submit that it is both at the same time.
If you look, the 5 Star ratings are "anonymous" ratings, not reviews. It is almost as if the raters knew there weren't words to justify the 5 Star rating.
Likewise, many of the 1 Star reviews probably should be 2 Stars - if the reviewer could set aside their feelings about the company / product.
The 4 Star reviews I read mentioned, and then quickly glossed over, the editing and production issues. Someone might be able to sell me on the idea that those should be 3.5 Stars, but DriveThuRPG won't allow that - so the reviewer rounded to 4. Maybe for some of them... Some should probably be brought down to 3.
The overall effect would be roughly the same, just more indicative of the quality of the product.
If you set aside your feelings, hopes, aspirations, and such and try to read the book objectively (I admit that it can be difficult) it reads kind of like someone took their college roommates hand written notes, typed them up, and submitted them as their thesis without reading them first.
There are quite a few good ideas in the Sixth World Core Book. I dare say there are a few great ideas in it. Not a one is fully fleshed out though.
There is a rumor that the playtesters tried to tell CGL that Shadowrun Sixth World needed another year abouts before it was ready. I can believe it.
It wouldn't hurt if they hired - and put under NDA if they needed - someone that wasn't entrenched in the design process to read through it to point out all the gaps of information.