Yeah the action is great, no doubting that. What generally sets Mission Impossible apart from say 007 IMO is the deception (i.e., the masks) and the plot's twists and turns ... I just don't think there was enough of that here.
Also that scene at the opera didn't really do it for me.. nice music and pretty unique but just wasn't interesting enough.
It's in the "Good but not Great" basket for me unfortunately.
Every action movie is just meh after the bar Mad Max set.