I have a large wrist and the new ones actually fit me better. The older model always popped up off center on me.
I had no problems with the new ones coming off (I did have an old one randomingly come off on EE a couple years ago).
No problems scanning on the new but only had one mess up on me before (the replacement band for the one lost to the Yeti) and it worked in the parks but didn't let me in the room so it had to be 'fixed'. Easy Peasy.