"The form submission cannot be processed because it exceeded the maximum length allowed by the Web administrator. Please resubmit"
By default, SharePoint web applications have limitation on maximum upload size for documents. If source has files bigger than 50Mb you will get error messages for them. The same error you would receive if tried to upload files within the browser.
You need manually change Maximum Upload size in Central Administrator:
1. Go to Central Administration > Application Management > Web Application General Settings
2. Select required web application and change the "Maximum Upload Size" value
3. Re-migrate documents that are failed.
Some additional changes in web application config file may be required. Please refer to KB925086