Since Marketing Cloud does not provide a quality environment, a possible workaround when using it connected to a Salesforce org (via Marketing Cloud Connect) could be to configure it as multi-org and use a business unit as Marketing Cloud quality, to be connected with the Salesforce quality org.
The documentation states a few constraints though. The most worrying stated in the public documentation is probably that Marketing Cloud business units are not data-isolated. Test data and production data are co-mingled when using a sandbox, which impacts data cleanliness and adds complexity to reporting, audience creation, etc.
An other one, that I cannot actually fully understand, is that email personalization is supported for standard fields only. Using custom fields from the Sales Cloud will require AMPScript. What are the standard field that the documentation is referring to?
More in general: do you suggest this approach to manage a quality marketing cloud environment or not given all the contraints and side effect? Thank you