9663d683_Sohel_s_Blog__Code_Activity
[Options/Matrix] Scalability [WCF Service] Since it’s service, you can scale it anyway, like workflow manager and SharePoint server can be hosted in on-premise, cloud or hybrid. [Code Activity] 
If you use Server Object model, you can’t move workflow manager to it’s own non-SharePoint Server. However if you use client object model, you can move workflow manager to it’s own non-SharePoint server.
Sohel's Blog
Code Activity