This doesn't address email routing. The targetaddress is what's used to route email, so having a different targetaddress for external cloud vs on prem users means that mail sent to cloud based mailboxes will be delivered directly to the cloud, bypassing any hygiene solutions tied to the mx records if mail is supposed to be routed on premises first. The way to get around this is with address rewrites, but this such a complicated solution where an autodiscover redirect and getting a new token would be much simpler. Also, you have to sync contacts between orgs, whereas without a hybrid cloud setup, this wasn't necessary.
Microsoft
