I can understand why someone would dislike the Cell Arc. It's the least impactful to the story as far as Z is concerned, and the Human Z Fighters finally take a backseat.
With that said, it's the Boo Arc where things really fall apart in my personal opinion. The power chains, which are one of Z's strongest points, are a complete mess in the early Boo Arc. Gohan becomes a joke until he becomes Ultimate/Mystic, Piccolo goes from a Z Fighter to a Mentor or just a babysitter, and Majin Vegeta was backwards character development (although some may argue it needed to happen). Don't even get me started on Trunks and Goten.
Boo is a bad villain in all of his forms as well. As Fat Boo you can't take him seriously, and Evil Boo/Super Boo is a more serious version of Fat Boo, but is still not a proper villain by any means. Bootenks and Boohan are poor knock offs of Perfect Cell, and Kid Boo has no real character.