I've implemented multi-org before and it is fairly straightforward. You need to follow the help documentation regarding Marketing Cloud Connector and you should be able to integrate a Salesforce Orgs to each Business Unit.
Once multi-org is enabled, you can't undo it. That should not be a concern if you are going to use more than 1 Salesforce Orgs. The only thing I can say is that there will be some challenges initially. I've had to work with Salesforce support to resolve issues like Tracking information not going to Sales Cloud, Queries unable to access Data extensions and with Journey Builder not having Salesforce activities visible.
I've also had issues after a Marketing Cloud release when Integration stops working and you have to login to Sales Cloud and enter the API user credentials in the Integration to make it work again.
Each set up is different and I believe you'll face a different set of challenges while you work on it. That being said, move forward with multi-org, coz that's the only option :) Good Luck