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.
Strange enough, this is not a new problem. For some users, it's been present ever since the Pixel and Pixel XL were released, back in October.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 PolicencG1vNJzZmivp6x7sbTOp5yaqpWjrm%2BvzqZmp52nqHyIu86go55lgJ7FpriMnZitmV2XrqS31KlkoqujqrKgtcNyamtvaA%3D%3D