Great answer by @bigassforce!
Disclaimer: I have not used S2S before but I spent 10 minutes skimming the docs and came to the conclusion it is not an enterprise grade product and I wouldn’t be surprised if Salesforce hides it in the attic soon.
Multi-master replication(also called master-master replication) is a hard engineering problem even just at the RDBMS layer. Read this doc for a high level view of the complexities and algorithms involved. There are also numerous hackernews articles that go into finer details of this.
Now, on top of this we have all the Salesforce specific application layer quirks and customer customisations to contend with. It would be a very hard engineering task to build a turn-key product that is general enough to work for most Enterprise scale customers (and their customisations)
I would recommend you do a thorough gap-fit analysis before jumping in.
Some considerations I would include as part of the Gap-fit analysis:
- Record Type(and their Picklist maps) mismatches between orgs
- Delete and Undelete complexities
- OWD mismatch between the orgs, Sharing(Implicit or explicit) and Group Maintenance
- Territory Management and associated sharing intricacies
- Approval Processes (and the associated child ProcessInstance intricacies )
- High complexity Managed packages such as CPQ that have heavy integration with objects such as Opportunity
- Field Type changes and how it affects already published data
- Chatter Feeds
- Shield Encryption
- Partner Accounts, Portal Roles and intricacies around it
- Person Accounts and complexities around it
- Contact to multiple Accounts and its usual complexities
- Account Merging
- Deduplication Rules
- Account and Opp teams (and default team assignments)
- Collaborative Forecasting and its intricacies with Opty fields and team splits