This confirms my decision. I'm sure it will be nice once it's all sorted out. The money quote for me is below:
Since our goal was to represent any .NET platform we had to break it up into smaller NuGet packages. This works reasonably well if all these components can be deployed with the application because you can update them independently. However, when you target an abstract specification, such as PCLs or the .NET Standard, this story doesn’t work so well because there is a very specific combination of versions that will allow you to run on the right set of platforms. In order to avoid that issue, we’ve defined .NET Standard as a single NuGet package.