In UDM 8.1, the 'Company' column was removed from the UDM spreadsheets making it impossible to migrate data into the system for all multi-tenant companies within a single UDM job, unless the backend is customised.
The workload and project timescale consequence of this change is especially notable when, for example, migrating Knowledge Articles from a legacy system which supports 50 companies - each RKM UDM dataset must be self-contained within its own spreadsheet and configured as 50 separate jobs within the UDM application. The same applies to People and other loads.
UDM naturally requires multiple iterations of "load -> validate -> correct -> load -> validate -> correct", until the data is correct and can be promoted into a Production system - with the UDM jobs split into individual companies, following this change in 8.1, hundreds of job creation iterations would be required for the scenario of 50 companies in a multi-tenant system.
In terms of security (the justification for this change to UDM), this would be far better controlled by UDM referencing the user's 'Access Restrictions' controlled within the 'People' form, that way granular control is permitted over who a user can and cannot load data for. In addition and from a security perspective, only a user with a DMT permission can load data, therefore the system is already secure.
For a go-live specifically which requires a downtime, this change in 8.1 dramatically extends the period of time required, in addition to the workload to create and manage separate dataload spreadsheets.