For our enrollment process of customers in Office 365, we enable the Unified Audit Log by default, as this has a great number of benefits.

At first, the only way to do this was through the UI, but now we can enable the Unified Audit Log with the following  Powershell command in the Exchange Online Remote Shell:

Set-AdminAuditLogConfig -UnifiedAuditLogIngestionEnabled:$True

Or to disable the Unified Audit Log through Powershell:

Set-AdminAuditLogConfig -UnifiedAuditLogIngestionEnabled:$False

Note that disabling the Unified Audit Log can only be done through Powershell.