What possible issues should I be aware of when migrating to Office 365?
There are two issues you should be aware of:
By default, Office 365 has a 25MB per item limit. From a practical perspective, the limits are a bit lower due to protocol overhead and encoding. As a result, items larger than 20MB in size may fail to migrate. However, you can increase this limit by following the instructions in this blog post from Microsoft.
Office 365 imposes a limit of 10 connections per admin account. If you're submitting more than 10 mailboxes concurrently, you may run into throttling-related errors such as "Connection did not succeed" and poor performance. You may
use impersonation as a way to resolve this problem.