I'm currently in the process of my first Sandbox <> Sandbox integration, and things are going well. As long as the configuration is mimicked between the SF sandbox and production portals, you shouldn't have any issues. The only labor-intensive piece is having to set up the mappings two times (once for sandbox, once for production), and there's obviously a chance of error there since it's a manual task.
If you have any specific questions, don't hesitate to reach out. jay@trujay.com
How can I connect my Hubspot sandbox to my Salesforce sandbox?
Is there a KB article that walks you through how to integrate HubSpot sandbox and Salesforce sandbox? Or is it exact the same steps as production version?