As already mentioned, SMTP Gateway to Exchange Online is one issue, especially in smaller environments. Not every SMTP client (printers etc.) is ready for TLS1.2, so these customers need not to replace that systems as soon they are uninstalling the last exchange server. (I know you should replace them, but customers do not recognize and some don't want to hear it). Another point is, in systems with only one admin, nobody understands why and how you should manage AD attributes. There are no PowerShell skills available and these admins hardly can handle the Exchange GUI / Exchange Online GUI. A lot of things can be done inside Exchange Online, so why not changing or adding SMTP Addresses. I'm consultant for this, I know how to manage that but people who need to administer many different systems will probably not understand these (little) complex symbiosis of local AD and Exchange Online. Thanks for your work and maybe there will be a smooth way to make this great E-Mail system a little handsome.
Thanks for this great blog and your work Exchange Team.