Simple Ultra and PeekABU are built on the same underlying diaper. Similarly, Simples, Space, and Little Paws are built on the same underlying diaper. I'm the opposite of you -- love the fit of the Simples, hate the fit of the Simple Ultra.
I'm sure there are things that irritate me, but not enough that I know what they are off the top of my head. And that's pretty nice -- not sure I want to start thinking about what bugs me.