One unfortunate discovery after transitioning our setup to take advantage of Subgroups, is that a Subgroup cannot have its own custom statuses.
In the same way that Groups inherit the System-wide statuses and can have additional custom statuses; Sub-groups should inherit their parent Group's statuses, and have the ability for their own additional statuses.
When we have a small team that is part of a large group that needs custom statuses, we have to choose to either make them a separate group (which introduces other challenges), or make their collection of custom statuses available to the larger group, which gets very messy and introduces confusion.