You need to specify a AMD compatible class, for instance a Dojo/Dijit widget. There is no support for user controls. There is legacy support for Web controls but this is something we want to phase out and requires a popup to work so it's nothing that we recommend for new development.
Ok, thank you for the information!
Let's say I want to change how a property is rendered in edit mode.
If I build my own editor descriptor, is the only way I can change the layout by setting ClientEditingClass and specifying a Dojo-file? Or can I use an UserControl?