Limitations of External Lists in SharePoint 2010

We can't compare external lists with standard lists where data is stored inside of SharePoint. Since the data is stored outside SharePoint, there are many Limitations in External Lists. Many of the Ribbon features will not be available for external Lists.

1. Cannot create Information Management Policies (bar codes,  auditing, retention or labels)
2. Open with Access/Open with Project is not supported
3. REST access via ListData.svc to External Lists is not available (All WCF services like RSS feeds are not available)
4. No Look ups
5. No version or version history
6. Inline Editing is not available
7. Datasheet view is not available
8. Export to Excel option is not available
9. Workflows cannot be configured for External Lists
10. Can't create Visio Diagrams
11. No Context Menu (ECB) Send To operations.
12. Events handlers are not supported.
13. No Item Level Permissions
14. No item or field level validation or Formula supported
15. Document templates are not supported.
16. Additional data connections in InfoPath 2010 list forms not supported
17. My favorite Utility: U2U CAML Query builder doesn't work!
18. No Check In / Check Out options 
19. Drafts of items are not supported.
20. No Write support for BLOB 
21. Metadata based navigation is not supported
22. Alerts are not supported
23. Attachments will not be available

Tail: You cannot mix the SharePoint columns with External List columns since they are coming from external source.

Read more: http://www.sharepointdiary.com/2012/03/limitations-of-external-lists-in.html#ixzz2I1zMavOw