Perhaps one of the most useful automated processes out there is the ability to do approval processes. We fortunately have two tools on-prem or online that allows us to perform this action. Microsoft Flow offers some incredible connectivity between services (like approve a Tweet and post it, approve something from Google Docs and have it moved to SharePoint, etc.), but the approval process itself is very simple at this point and doesn’t offer some of the more robust features and customization options we get in SharePoint Designer 2013 approval processes.
I also will use both tools in the same business process occasionally, because they both have unique strengths.
But which do you use for approvals?
The quick answer to the question is: Use Flow for simple approvals, or approvals that involve multiple sites or external services. Use SPD for more complicated processes and customization options for approvals that involve a single site.