This is probably a little bit niche, and could just be my code rather than this awesome project, but I've found that if my mapping delegate returns a generic property then it doesn't work. E.g.

public interface IMapClass{
        CustomMapping GetCustomMapping();
}

public class GenericTypeDefClass<T> : IMapClass where T is class, new(){
    public CustomMapping GetCustomMapping()
        {
            return MapProperty;
        }

        public static T MapProperty(IUmbracoMapper mapper, IPublishedContent contentToMapFrom, string propertyName, bool recursive)
        {
            return contentToMapFrom.GetPropertyValue<T>(propertyName, recursive, null);
        }
}

//Create a type for GenericTypeDefClass<string>
Type newType = typeof(GenericTypeDefClass<T>).MakeGenericType(typeof(string));
//Create an instance of GenericTypeDefClass<string>
IMapClass c = (IMapClass)Activator.CreateInstance(type);
//Get the delegate
var mappingDelegate = c.GetCustomMapping();
//arguments are typeof(string).FullName and the MapProperty delegate where T is string
Mapper.AddCustomMapping(mappingDelegate.Method.ReturnType.FullName, mappingDelegate);

I can imagine this isn't a problem that many people are coming across, but perhaps you can identify if there is a way around this issue?

Of course, this is actually a moot point if my other post has a solution...