Our customer (a FMCG manufacturer) is using Tally for accounting purpose currently, and is willing to integrate ERPNext (to automate accounting of Sales,Manufacturing, procurement etc.,). We are evaluating between the following approaches .
Keep ERPNext as the master accounting system, and replace Tally going forward. We are not sure if there are any delta functionalities of Tally, that ERPNext cannot support.
Keep Tally as the master accounting system, but feed the accounting data from ERPNext platform realtime (through Tally API calls).
Not sure on what basis should we decide one of the above two approaches. Are there any other suggestions. Customer does not have any preference yet, and are open to our suggestions.
I think you should go with the First Approach because ERPNext has all the functionality which is available in Tally. Also Tally Migration Integration is available out of the box, you have to look into it.
On the contrary you will find more features that Tally does not support which ERPNext handles on business as usual basis. Plus ERPNext has excellent change tracking mechanism that Tally lacks. System and database security wise there is no comparison between both system.
Integration will be difficult as you need 2 way sync which is currently not supported. Development cost and time would be very high. Better for your client to close books in Tally and make it as opening entry in ERPNext.