With the new Unified Interface apps for Dynamics 365, it is now possible to "log in" as another user. This will be a major benefit to administrators/customizers as they troubleshoot issues on behalf of their users. The reason this is possible is because most (all?) of the data interactions in Unified Interface apps are done through the API, and the API supports user impersonation!
How do you do it? Essentially, we just need to add the MSCRMCallerID header to every API request. One way to do this is using the "ModHeader" extension for Google Chrome. Once you have it installed, create a new profile to add the header to all requests for your environment. It should look something like this...the MSCRMCallerID is the GUID of the user you want to impersonate. There are a few ways to get this, but my favorite is navigating to the user record and using the "Record ID" action from the Level Up extension by Natraj Yegnaraman.
That's it! If you refresh your Unified Interface app, you should notice the user name in the upper right corner change, and all requests will be filtered based on the security context of the user you are impersonating! If you create any records, it will show as being created by the impersonated user, with a delegated created by of your user.
Although most features seem to work in the context of the impersonated user, some don't appear to. For example, Relevance Search -- it will return the results based on your user, not the impersonated user.
This was first proposed as an idea over 7 years ago, and it's currently planned for development in the mid-term. While we wait for Microsoft to build it in to the platform, we at least have a solution that (mostly) works!