Active Directory Sites should be unique on something more than just name
Currently, if you have forests that have the same Active Directory Site name in them, SCCM only imports the first one it finds as a boundary. This makes using a Site as a boundary very unreliable when assigning site systems.
Forest a.com has a Site 'HQ'
Forest b.com also has a Site 'HQ'
SCCM imports a.com/HQ as AD Site, ignoring b.com/HQ
Client pc1.b.com is in Site HQ in b.com, but due to the boundary mixup, tries to access the DP in a.com, which it doesn't trust.
If Sites were made to be unique in a "(domain or forest)/Site" (or some other manner that includes more than just site name) rather than just Site name, pc1.b.com would know to get to the DP in b.com's network instead.