Multitenancy exists on a spectrum of isolation and can vary depending on the client's needs.
At one end is no isolation, usually seen in implementations by unskilled consulting shops, where everyone shares an instance and there are folders for different groups to use. At the other end is total isolation, where users don't even know other tenants exist on the instance and there is strong governance for releases and administration.
I've generally seen clients want either total isolation or a softer form of it with sharing for some users.
Pros are it's cheaper, cons are that it's more complex and less secure- shared db user underpins tenants, privilege escalation is more impactful.
Multitenancy is largely undocumented and has assorted limits, all of which can be overcome.
Look into data schema, form, and jssp conditionals, along with which ootb folders need which permissions for the new groups you're creating. Also have an overall vision of folder structure, user groups, and overall governance of how the instance is used and deployed.
FWIW I have a drop-in module that can be configured for varying degrees of tenant isolation, which Adobe should have an earlier iteration of in their ACX repo.
What I am trying to understand is what does multi-tenancy means in Campaign Classic. what are its impact on data partitioning, OOTB reports, templates etc. I have just started doing the research and trying to understand the pros and cons of multi-tenancy.
I believe in Campaign Standard, upgrade can happen at the same for all the tenants. How does it work in Campaign Classic ?