You may check note 1446253 question 6.
You need to replicate products from ECC to CRM.
Same for BP/customer in actual fact.
The most commonly used scenario is to create ERP quotation from Opportunity.
If the product or BP is not replicated to CRM which means, the products and BPs in Opportunity only exist in CRM, then the copy from Opportunity to ERP quotation will fail.
This is the major impact.
As a result, you need to replicate the products to CRM though you can now enter the product ID in ERP quotation w/o any problem yet.
Spencer