Do you have any particular experiences with creating users’ email addresses when migrating to Confluence Cloud that you can share? We have encountered some issues or challenges during a test migration with the email addresses of users.
Yes, we already have some experiences from our Confluence cloud test migration that we would like to share with everyone: When creating or matching users in the cloud, the mail address is used as the only criteria. Normally this shouldn’t be a problem, but if there are supposedly 
fictitious mail addresses in the source instance, something like “test@test.com” or “mail@example.com”, Atlassian also simply matches that with all accounts existing in the cloud.
We suddenly had a completely unknown user in our Cloud Confluence (and also as an admin), who apparently managed to click an account with admin@example.com as a mail address at Atlassian (which is exactly what we also use in some tests). At the moment I don’t know how to make sure that only users with mail addresses of a certain domain are imported/matched during the migration. So you should keep that in mind.
There is another aspect: If you work in systems with test users, they often all have one mail address (real or fictitious). They might all become one single user during the transfer (I haven’t tested this yet).