According to various Pixel and Pixel XL users, their devices have trouble backing up data like SMS, call history, apps, and so on - there's a "Waiting to backup" status that won't go away no matter what, as seen in the screenshot on the left.
As of this week, Google is still "actively looking" into the backup issue. Hopefully, a fix will be found soon. Meanwhile, you may not even know that your Pixel isn't properly backing up your data. Fortunately, you can manually check to see if things are all right in this regard by going to Settings -> Backup and Reset -> Backup.
sources: Google Forums, Reddit via Android Police