I had issues with custom extension which was using custom tables with serialized fields
In Magento 2.1 default php serialize function is used.
In Magento 2.2 json serialize is used and trying to unserialize old entites will end up with error
Error: Unable to unserialize value.
You need to check your custom modules if you are extending default Magento functionalities or overriding classes using <preference>
Magento official guide to convert serialized data to 2.2 Thanks to @Swapnil Tatkondawar https://devdocs.magento.com/guides/v2.2/ext-best-practices/tutorials/serialized-to-json-data-upgrade.html