Challenges to be considered before migrating from SharePoint 2010 to 2016

1. Identifying the business owners for each site / site collection
2. Data Cleansing - identifying the old contents/obsolete data for archiving
3. Identifying the intranet links embedded inside the documents (while migrating to SharePoint Online) 
4. Optimize and define databases - Perform re-organization of content. For example, as you determine what will be brought over to the new environment, you may end up archiving some ‘less-used’ content to a ‘read-only’ or ‘slower’ server.Consider moving read only data to a separate SQL instance or even separate SQL farm where the data is read-only. You may also want to consider moving less frequently used content to SQL servers that have less resources and keep the higher performing SQL servers for the frequently used/collaborative content, perhaps your ‘old’ SQL server.
5. Size of data to be migrated – this will affect the migration time
6. Any good migration should involve a content audit. Data security (legal & audit implications should be considered) –confidential documents which are restricted for internal use cannot be migrated to SharePoint Online. Key ‘things’ like ‘Highly Visible or Critical Areas’, ‘Executive Users Permissions’, ‘Large AD Group Users’ assignments, ‘Highly Used’ Features, ‘Heavy Customizations’, Integration with other systems, etc. must be identified, and documented in order to identify where the critical data and functionality exists. Depending on the access to the content, an appropriate ‘services’ and ‘security’ model can be included in the SharePoint migration planning (and later validation).
7. SharePoint 2010 cannot be directly migrated to 2016 and it needs to be migrated to 2013 and then to 2016 version (when Database upgrade approach is opted as an option for migration)
8. If tool based migration (like Sharegate, Metalogix) approach is used, then direct migration from 2010 to 2016 is possible
9. 10 GB is the new soft limit for file sizes in SharePoint 2016
10. If custom master page is used, then the existing master page needs to be upgraded /recreated from 2010 -2013 –then 2016 version for using it in SharePoint 2016
11. While using database upgrade (2010 to 2013 and then to 2016) approach, SharePoint 2010 sites will be made read only during migration process
12. Any custom farm solutions should be upgraded to SharePoint 2016 before use
13. The same connectivity/integrations to external systems in SharePoint on-premises environment may not be achieved in SharePoint Online
14. New user ID’s needs to be created in SharePoint Online. New/ existing permission levels needs to be created in SharePoint Online
15. End users needs to be trained for new UI/navigation, etc.