Exchange Online, with the version 15 tenant of Office 365, now supports public folders.
If your organization is using public folders, this factor requires additional planning and extra steps in the migration.
Public folders in Exchange Online have a fundamentally different architecture to those in other versions of Exchange.
In essence, Exchange Online holds public folder data in a series of mailboxes.
The first public folder mailbox you create holds the folder hierarchy while the remaining mailboxes store the data.