Someone recently asked me what I thought of someone adding 1,000 + business units to Dynamics 365. I told them it was a bad idea. Here’s why:
Business units are like large granite rocks–they are designed to be permanent and infrequently moved. While users can be moved between business units, it is not a trivial matter, especially if they own many records.
When you move a user from BU 1 to BU 2, the business unit association of every record that user owns changes. This can cause some surprises to other users who are members of the user’s original business unit if they have BU level read permission. The records owned by the moved user are now not available to them, but if they own child records of those records, like activities, it can cause some strange scenarios. Also, if the user owns many records, moving users between business units can be time-consuming.
Another potential impact from large quantities of business units is security role updates. Each role is not just one record–a copy of each role is added to each business unit. So if you create thousands of business units, making a small change to a security role can take hours.
My recommendation is to keep your business units to a minimum–only the minimal number to facilitate true BU security requirements. For more granular user segmentation, consider the use of teams. Teams are much more flexible, they can be used to control security access to records, and users can be members of multiple teams.