Some users have been experiencing issues with backup on Google's Pixel devices, most notably backups of SMS, call history, and apps, although other accounts of all backup types failing have also been reported. The current cause isn't known, but reports of the problem on Google's product forums stretch back to just after the launch of the device, and for many the matter remains unresolved.
Even one of Google's support specialists discovered he was experiencing the issue, but apart from reaching out to those affected for bug reports no information has been publicly provided. To be clear, for those with the problem their personal data in one or more categories may not be being backed up as they would expect. Not to be alarmist, but with this backup error occurring silently, it might be prudent for Pixel users to check that their devices are, in fact, performing their backups as expected and intended.
To check for yourself, navigate to Settings > Backup and Reset > Backup and see if any of the items listed under Content are "Waiting to backup." If you are among those suffering the problem you might want to exercise some caution and consider another backup method for whatever might be failing for you. Also, feel free to add your voices (and log-files) down at Google's product forums. They're currently reaching out to those affected there for additional information, and with more data the issue can probably be fixed that much more quickly.