When the "old" 5 Series first came out, they had issues. They experimented with things. The new "Next Generation" 5 Series is awesome! It's like they worked all the bugs out, and got it right.
When the 8 Series first came out, it was a disaster. They had dealers replacing 20 parts, rotated the hook 30 degrees, re-designed the bobbin case and had repeated firmware updates. The new 880s are fixed, but they're so over-engineered, you have to be a rocket scientist to run it.
The 7 Series is the product of all of that trial and error. The 790 is my dream machine! I had to sew 12 layers of denim at the store on Monday, and it did it without a hitch.