There are some adapters that you want running on a Clustered Host Instance e.g. that is only running on a single node a time, as these adapters have no mechanism to prevent picking the messages up at the same time.
These include
- POP3 (possibly also the new Office 365 Mail adapter)
- WCF-SQL
- FTP(s)
- SFTP
- MSMQ
- SAP
- receive adapter that supports ordered delivery