If you already have the Telerik.UI.for.Xamarin NuGet package installed to your projects and you use the Telerik Toolbox to add a control, it will break your references by duplicating references to the same assemblies (one for newly added assembly reference, one for existing package reference).
When you drag and drop in a control from the Toolbox, it will add the assembly references that point to the installation folder, even if I already have the NuGet Package referenced. This means that you would need to manually have to remove the Telerik assembly references every time you use the Toolbox. The problem is exasperated if the project structure doesn't support package reference (i.e. you cant tell which assembly to remove)
Ultimately, this means you can't use the Telerik UI for Xamarin Toolbox when using NuGet packages.