I can shed some view points on service contract.
CRM contract is more flexible than ECC contract.
The componets like Action are not available in ECC. So the follow up activity such as contract renewal requires manual intervention
CRM contract can be billed either in CRM or ECC. But ECC contract can be billed in ECC only
The billing capabilities of CRM & ECC is different. So it will have its own impact on the Contracts.
If CRM contract is not replicated to ECC, it requires a CO order for the financial integration (depends on the Financial integration.)
Hope it will help you