There are several reasons enrolling devices directly to the top-level organization group (OG), commonly known as Global, is not a good idea. These reasons are multitenancy, inheritance, and functionality.
You can make as many child organization groups as you need and you configure each one independently from the others. Settings you apply to a child OG do not impact other siblings.
Changes made to a parent level OG apply to the children. Conversely, changes made to a child level OG do not apply to the parent or siblings.
There are settings and functionality that are only configurable to Customer type organization groups. These include wipe protection, telecom, and personal content. Devices added directly to the top-level Global OG are excluded from these settings and functionality.
The Global organization group (OG) is designed to house Customer and other types of OGs. Given the way inheritance works, if you add devices to Global and configure Global with settings intended to affect those devices, you are also affecting all the Customer OGs underneath. This undermines the benefits of multitenancy and inheritance.