Some people had issues with the fact that each episode focused on a single member of the family and, at first, I too had concerns. However, the way it was done was perfect! My only issue is that I felt the series could have benefited from a family focused episode for the finale that tied everything together. That's not to say it didn't come together but it was odd having it all resolve through an episode dedicated to George Michael.
My only other issue is that I felt the portrayal of Michael Bluth didn't quite fit the character from previous seasons. He seemed a little too desperate at times and, in contrast, overly cocky at other moments. Michael is supposed to be the mild mannered character that comparatively seems normal but that wasn't the case throughout the Netflix series. Maybe that was intentional but it felt a bit off character.
Otherwise I thought the series was fantastic. The writing was brilliant and I might even say the laughs came at a faster pace than previous seasons. I don't know if they plan on another season but I would love to see it. Although, it would be better to release each episode on a weekly schedule. I think the reason for the negative reviews was solely a matter of people powering through all the episodes in a single sitting and not letting things sink in.