I wouldn't say it's their ability that's stopping them from having harder dances; they're all very capable dancers. Some weaker than others, some very strong, yeah, but they're all capable of doing something more challenging, and doing it well. Golden Chinatown and Sayonara Usotsuki no Watashi had pretty good dances in my opinion, definitely stronger than some of their previous routines, and they said themselves they worked hard on the synchronicity for these. However, the next single's dances seemed weaker, and now it's like the choreographer's tried something a bit out of the box and a bit weird, but for the most part, it's just not working that well.
Anyway, I'm hoping that with them having only recently recorded the MV for Diet, they've had chance to practice synchronising better; to me it looked like this dance needs to be synched well to look good and in the performance we saw, it just wasn't.