I have just updated to RC1, and went directly to Version 3.14. I had the same basic issues as others as regards the change to overlays, etc, but I got most of the app to work. I'm doing all of my work with custom FeatureSources and custom FeatureLayers.
In the initial conversion, I got some base layers to draw, but could never get one special layer to draw. I decided to update the IsVisible logic on the layers by using the new Override that you provided. Now, with just that one change to my layer implementation (and yes, IsVisible is returning true), I no longer get any maps to draw. I've put a breakpoint in the IsVisible_Get property, and it is being hit.
You did not provide an IsVisibleCore override... just IsVisible. Do I need to be calling base.IsVisible for any reason within my override?
I have also put a breakpoint in my custom FeatureSource.GetAllFeaturesCore, and I am correctly returning a Features collection with the right count for each of the layers that should be drawing.
At this point, I don't have any idea how to proceed.