Hi Umesh,
As I mentioned in my first response, in all the FMCG in which I worked, Finished and Semifinished goods are planned in APO. APO is a more advanced planning tool: you can run heuristics, combine different heuristics, run optimizer, consider different restrictions, etc.. So Processes as Demand Planning, SNP, PPDS and Deployments are managed in APO. In ECC you would have only the MRP for components (components not managed in APO).
This is what I saw in the companies in which I was part of the implementation teams.
You would need to evaluate what is more convenient for your client. My advice would be: only use APO if you need more inteligence to resolved your planning. If something does not need a complex heuristic and can be planned with a simple MRP run, I would use ECC.
You mentioned open documents and master data changes. I think that if you have those problems in ECC, in APO the situation can be even worst.
Thanks and Regards,
Mariano