He's had his ups and down but overall he's probably been fine. I think we'd like to have seen more consistency out of him but the gaffes he makes are more noticeable than the stuff he does well most of the time, which is why the eye test is always flawed in its own ways. Our brains definitely remember those moments that stand out far more than it recognizes some other baseline of consistency
Throwing on my BS speculating hat I think Miller really wants to be a go to star and perhaps has pushed too hard and tried tdo play above his level at times. His skating ability is a great strength, he can carry the puck quite well, but he can't make elite level passes and shouldn't try to, and he needs to watch his pinches because he gets caught on those too often. To some extent, let the game come to him when it matters and use his best attributes to push the pace when appropriate. I think he arguably pushed too hard this season trying to make it a big breakout year