Business Connectivity Services Limitations

Even though BCS is great and can do wonders when you want SharePoint 2010 to interact with your external systems or other LOB applications, it has its own limitations. Understanding these limitations will help you build good BCS solutions.
Below are some/all of the BCS/External Lists limitations:

Workflows cannot be associated with external lists

However, you can use the external data columns and manipulate the external lists in a workflow.

No RSS feed support for external lists.

No REST based access for external lists

The http://site/_vti_bin/ListData.svc does not support external lists

LINQ to SharePoint spmetal.exe does not support external lists

Cannot configure alerts for external lists

Cannot export external list items to Excel (Export to Excel feature)

Item-Level permissions is not available for external lists

Versioning cannot be configured on external lists

Item History is not available on external lists

Datasheet view cannot be used in external lists

XSLT is supported though

You cannot create a site column of type ‘External Data Column’

External data columns can be created only as list based columns and cannot be consumed in site level content types

No Write support for BLOB

You cannot write back to BLOB fields using BCS unless you write your own method

You can access BLOB columns by defining a StreamAccessor method and presenting the external data via the BCS Data List web part by checking the Display stream fields property

Ratings feature is not supported for external lists

Metadata based navigation is not supported for External Lists

Some of the limitations are quite obvious as the data does not reside in SharePoint and it is not SharePoint’s responsibility on how the data behaves. SharePoint is here to just present the external data. That said, I do think some are not limitations, but Microsoft just didn’t have time to implement, like:

RSS feed support

External data site columns

Export to Excel

Can’t connect data to workflows

There is no way for you to run a workflow on records contained in external lists.  In SharePoint 2010 you can only attach workflows to documents, list items and content types – not external lists.  You could map external list records to SharePoint lists and connect a workflow to the internal list, but think about what’s needed to sync the two lists.                                                                                                                                                                   
Lack of record-level permissions
This means you can’t control who has access to specific records in the external list – like marketing staff being able to view but not change entries belonging to sales.                                                                            
No REST access
You cannot get external data that sits somewhere out on the web, like SaleForce.com.
 
No attachment support
SharePoint BCS 2010 does not allow for attachments, so you can’t link a record to a relevant document.  Well, you might be able to store the attachment in the external database, but read on…
 
No BLOB write support
You can’t write to a BLOB field through BCS, unless you come up with your own write method.
 
Can’t export data to Excel
…or other popular file formats, but if the data has numbers in it users are going to be asking you how to get it into Excel (“cut and paste” wasn’t well received when I tried suggesting it). 
 
No datasheet view and inline editing
This means you cannot edit items directly when viewing them as a grid.  You need to open the individual item view to edit the contents.
 
Lack of field and record level validation
You can’t control what a user enters into the field without resorting to the intricacies of SharePoint Designer or InfoPath.
 
Limited lookup functionality
When you retrieve a value of “2732” in the Industry Code field of your CRM contact, that’s all you’re going to see since you can’t link the field to the Industry Codes table.  The ID column is the one thing that you can look up on external lists; otherwise you’ll have to start coding to be able to get “Book Printing” instead of a cryptic number.
 
Lack of alerts
You can’t configure alerts on external lists through BCS.
 
Lack of RSS feeds
When using BCS in SharePoint 2010, RSS feeds are not accessible.
 
Lack of ratings
One can’t rate items that are contained in an external list. 
 
No versioning
You can’t keep a version history of changes to external lists, but maybe you couldn’t in the original system ether…                                                            

Courtesy:http://pravahaminfo.blogspot.in/2012/01/business-connectivity-services.html http://go.limeleap.com/community/bid/239118/SharePoint-BCS-2010-Limitations-and-Challenges